Integrates the blockchain v2 reactor with state sync, fixes #4765. This mostly involves deferring fast syncing until after state sync completes. I tried a few different approaches, this was the least effort:

* `Reactor.events` is `nil` if no fast sync is in progress, in which case events are not dispatched - most importantly `AddPeer`.

* Accept status messages from unknown peers in the scheduler and register them as ready. On fast sync startup, broadcast status requests to all existing peers.

* When switching from state sync, first send a `bcResetState` message to the processor and scheduler to update their states - most importantly the initial block height.

* When fast sync completes, shut down event loop, scheduler and processor, and set `events` channel to `nil`.

Fixes #828. Adds state sync, as outlined in [ADR-053](https://github.com/tendermint/tendermint/blob/master/docs/architecture/adr-053-state-sync-prototype.md). See related PRs in Cosmos SDK (https://github.com/cosmos/cosmos-sdk/pull/5803) and Gaia (https://github.com/cosmos/gaia/pull/327).

This is split out of the previous PR #4645, and branched off of the ABCI interface in #4704. 

* Adds a new P2P reactor which exchanges snapshots with peers, and bootstraps an empty local node from remote snapshots when requested.

* Adds a new configuration section `[statesync]` that enables state sync and configures the light client. Also enables `statesync:info` logging by default.

* Integrates state sync into node startup. Does not support the v2 blockchain reactor, since it needs some reorganization to defer startup.

## Description

Fixes a bug where the reactor would broadcast a base with height=0.

______

For contributor use:

- [ ] Wrote tests
- [ ] Updated CHANGELOG_PENDING.md
- [ ] Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
- [ ] Updated relevant documentation (`docs/`) and code comments
- [x] Re-reviewed `Files changed` in the Github PR explorer

* Added BlockStore.DeleteBlock()

* Added initial block pruner prototype

* wip

* Added BlockStore.PruneBlocks()

* Added consensus setting for block pruning

* Added BlockStore base

* Error on replay if base does not have blocks

* Handle missing blocks when sending VoteSetMaj23Message

* Error message tweak

* Properly update blockstore state

* Error message fix again

* blockchain: ignore peer missing blocks

* Added FIXME

* Added test for block replay with truncated history

* Handle peer base in blockchain reactor

* Improved replay error handling

* Added tests for Store.PruneBlocks()

* Fix non-RPC handling of truncated block history

* Panic on missing block meta in needProofBlock()

* Updated changelog

* Handle truncated block history in RPC layer

* Added info about earliest block in /status RPC

* Reorder height and base in blockchain reactor messages

* Updated changelog

* Fix tests

* Appease linter

* Minor review fixes

* Non-empty BlockStores should always have base > 0

* Update code to assume base > 0 invariant

* Added blockstore tests for pruning to 0

* Make sure we don't prune below the current base

* Added BlockStore.Size()

* config: added retain_blocks recommendations

* Update v1 blockchain reactor to handle blockstore base

* Added state database pruning

* Propagate errors on missing validator sets

* Comment tweaks

* Improved error message

Co-Authored-By: Anton Kaliaev <anton.kalyaev@gmail.com>

* use ABCI field ResponseCommit.retain_height instead of retain-blocks config option

* remove State.RetainHeight, return value instead

* fix minor issues

* rename pruneHeights() to pruneBlocks()

* noop to fix GitHub borkage

Co-authored-by: Anton Kaliaev <anton.kalyaev@gmail.com>

The work includes the reactor which ties together all the seperate routines involved in the design of the blockchain v2 refactor. This PR replaces #4067 which got far too large and messy after a failed attempt to rebase.

## Commits:

* Blockchainv 2 reactor:

	+ I cleaner copy of the work done in #4067 which fell too far behind and was a nightmare to rebase.
	+ The work includes the reactor which ties together all the seperate routines involved in the design of the blockchain v2 refactor.

* fixes after merge

* reorder iIO interface methodset

* change iO -> IO

* panic before send nil block

* rename switchToConsensus -> trySwitchToConsensus

* rename tdState -> tmState

* Update blockchain/v2/reactor.go

Co-Authored-By: Bot from GolangCI <42910462+golangcibot@users.noreply.github.com>

* remove peer when it sends a block unsolicited

* check for not ready in markReceived

* fix error

* fix the pcFinished event

* typo fix

* add documentation for processor fields

* simplify time.Since

* try and make the linter happy

* some doc updates

* fix channel diagram

* Update adr-043-blockchain-riri-org.md

* panic on nil switch

* liting fixes

* account for nil block in bBlockResponseMessage

* panic on duplicate block enqueued by processor

* linting

* goimport reactor_test.go

Co-authored-by: Bot from GolangCI <42910462+golangcibot@users.noreply.github.com>
Co-authored-by: Anca Zamfir <ancazamfir@users.noreply.github.com>
Co-authored-by: Marko <marbar3778@yahoo.com>
Co-authored-by: Anton Kaliaev <anton.kalyaev@gmail.com>