This is another batch of things to cleanup in the legacy P2P system.

A few notes:

- this is not all the deletion that we can do, but this is the most
  "simple" case: it leaves in shims, and there's some trivial
  additional cleanup to the transport that can happen but that
  requires writing more code, and I wanted this to be easy to review
  above all else.
  
- This should land *after* we cut the branch for 0.35, but I'm
  anticipating that to happen soon, and I wanted to run this through
  CI.

This commit should be one of the first to land as part of the v0.36
cycle *after* cutting the 0.35 branch. 

The blocksync/v2 reactor was originally implemented as an experiement
to produce an implementation of the blockstack protocol that would be
easier to test and validate, but it was never appropriately
operationalized and this implementation was never fully debugged. When
the p2p layer was refactored as part of the 0.35 cycle, the v2
implementation was not refactored and it was left in the codebase but
not removed. This commit just removes all references to it.

The code in the Tendermint repository makes heavy use of import aliasing.
This is made necessary by our extensive reuse of common base package names, and
by repetition of similar names across different subdirectories.

Unfortunately we have not been very consistent about which packages we alias in
various circumstances, and the aliases we use vary. In the spirit of the advice
in the style guide and https://github.com/golang/go/wiki/CodeReviewComments#imports,
his change makes an effort to clean up and normalize import aliasing.

This change makes no API or behavioral changes. It is a pure cleanup intended
o help make the code more readable to developers (including myself) trying to
understand what is being imported where.

Only unexported names have been modified, and the changes were generated and
applied mechanically with gofmt -r and comby, respecting the lexical and
syntactic rules of Go.  Even so, I did not fix every inconsistency. Where the
changes would be too disruptive, I left it alone.

The principles I followed in this cleanup are:

- Remove aliases that restate the package name.
- Remove aliases where the base package name is unambiguous.
- Move overly-terse abbreviations from the import to the usage site.
- Fix lexical issues (remove underscores, remove capitalization).
- Fix import groupings to more closely match the style guide.
- Group blank (side-effecting) imports and ensure they are commented.
- Add aliases to multiple imports with the same base package name.

The 0.35 release cycle renamed the 'fastsync' functionality to 'blocksync'. This change brings the configuration parameters in line with that change. Namely, it updates the configuration file `[fastsync]` field to be `[blocksync]` and changes the command line flag and config file parameters `--fast-sync` and `fast-sync` to `--enable-block-sync` and `enable-block-sync` respectively.

Error messages were added to help users encountering these changes be able to quickly make the needed update to their files/scripts.

When using the old command line argument for fast-sync, the following is printed

```
./build/tendermint start --proxy-app=kvstore --consensus.create-empty-blocks=false --fast-sync=false
ERROR: invalid argument "false" for "--fast-sync" flag: --fast-sync has been deprecated, please use --enable-block-sync
```

When using one of the old config file parameters, the following is printed:
```
./build/tendermint start --proxy-app=kvstore --consensus.create-empty-blocks=false
ERROR: error in config file: a configuration parameter named 'fast-sync' was found in the configuration file. The 'fast-sync' parameter has been renamed to 'enable-block-sync', please update the 'fast-sync' field in your configuration file to 'enable-block-sync'
```

This is just a configuration change to default to using the new stack
unless explicitly disabled (e.g. `UseLegacy`) this renames the
configuration value and makes the configuration logic more clear.

The legacy option is good to retain as a fallback if the new stack has
issues operationally, but we should make sure that most of the time
we're using the new stack.

EDIT: Updated, see [comment below]( https://github.com/tendermint/tendermint/pull/6785#issuecomment-897793175)

This change adds a sketch of the `Debug` mode. 

This change adds a `Debug` struct to the node package. This `Debug` struct is intended to be created and started by a command in the `cmd` directory. The `Debug` struct runs the RPC server on the data directories: both the state store and the block store.

This change required a good deal of refactoring. Namely, a new `rpc.go` file was added to the `node` package. This file encapsulates functions for starting RPC servers used by nodes. A potential additional change is to further factor this code into shared code _in_ the `rpc` package. 

Minor API tweaks were also made that seemed appropriate such as the mechanism for fetching routes from the `rpc/core` package.

Additional work is required to register the `Debug` service as a command in the `cmd` directory but I am looking for feedback on if this direction seems appropriate before diving much further.

closes: #5908

* node: minimize hardcoded service initialization

* hacking

* nil safety

* reduce space

* remove genesis state store

* fix lint

* fix pex

* unwind some odering effects

* fix tests

* remove unused experiment

closes #2498 
solves part of #3365

Note: difficult to test the event emit in SwitchToFastSync part, might need to change `stateSyncReactor` to an interface in the `nodeImpl` struct

## Description

Remove v2 flag from toml

* docs: update events

* lint++

* lint++

## Description

Expose p2p functions for use in the sdk. 

These functions could also be copied over to the sdk. I dont have a preference of which is better.

Having smaller buffers in each reactor/channel will mean that there will be fewer stale messages.

use  `maxPeerBlockHeight` information to show the current network's best height.

Closes #3983
Relate to #3365

ref: the`highestBlock` in the response of `eth.isSyncing` call
https://web3js.readthedocs.io/en/v1.3.4/web3-eth.html#issyncing

Closes #3507

This PR make some tweaks to backfill after running e2e tests:
- Separates sync and backfill as two distinct processes that the node calls. The reason is because if sync fails then the node should fail but if backfill fails it is still possible to proceed.
- Removes peers who don't have the block at a height from the local peer list. As the process goes backwards if a node doesn't have a block at a height they're likely pruning blocks and thus they won't have any prior ones either. 
- Sleep when we've run out of peers, then try again.

state: move pruneBlocks function from consensus/state to state/execution

Closes #5414

Remove `Context` from the `TxInfo` type and instead require the caller to pass a `Context` to `CheckTx` which is idiomatic.

closes: #6497

Addresses a beginning component of #6255

## Description

Add version back to versions, but allow it to be overridden via a ldflag.

Reason:

Many users are not setting the ldflag causing issues with tooling that relies on it (cosmjs)

closes #6488

cc @webmaster128