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.

## Description

Remove v2 flag from toml

Implementation spec of Double Signing Risk Reduction [ADR-51](https://github.com/tendermint/tendermint/blob/master/docs/architecture/adr-051-double-signing-risk-reduction.md) by B-Harvest
- Add `DoubleSignCheckHeight` config variable to ConsensusConfig for "How many blocks looks back to check existence of the node's consensus votes when before joining consensus"
- Add `consensus.double_sign_check_height` to `config.toml` and `tendermint node` as flag for set `DoubleSignCheckHeight`
- Set default `consensus.double_sign_check_height` to `0`  ( it could be adjustable in this PR, disable when 0  )

Refs

- [ADR-51](https://github.com/tendermint/tendermint/blob/master/docs/architecture/adr-051-double-signing-risk-reduction.md)
- [https://github.com/tendermint/tendermint/issues/4059](https://github.com/tendermint/tendermint/issues/4059)
- [https://github.com/tendermint/tendermint/pull/4262](https://github.com/tendermint/tendermint/pull/4262)

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.

* 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>

* Add test for bcBlockRequestMessage ValidateBasic method

* Add test for bcNoBlockResponseMessage ValidateBasic method

* Add test for bcStatusRequestMessage ValidateBasic method

* Add test for bcStatusResponseMessage ValidateBasic method

* Add blockchain v1 reactor ValidateBasic tests

* Add test for NewRoundStepMessage ValidateBasic method

* Add test for NewValidBlockMessage ValidateBasic method

* Test BlockParts Size

* Import cmn package

* Add test for ProposalPOLMessage ValidateBasic method

* Add test for BlockPartMessage ValidateBasic method

* Add test for HasVoteMessage ValidateBasic method

* Add test for VoteSetMaj23Message ValidateBasic method

* Add test for VoteSetBitsMessage ValidateBasic method

* Fix linter errors

* Improve readability

* Add test for BaseConfig ValidateBasic method

* Add test for RPCConfig ValidateBasic method

* Add test for P2PConfig ValidateBasic method

* Add test for MempoolConfig ValidateBasic method

* Add test for FastSyncConfig ValidateBasic method

* Add test for ConsensusConfig ValidateBasic method

* Add test for InstrumentationConfig ValidateBasic method

* Add test for BlockID ValidateBasic method

* Add test for SignedHeader ValidateBasic method

* Add test for MockGoodEvidence and MockBadEvidence ValidateBasic methods

* Remove debug logging

Co-Authored-By: Marko <marbar3778@yahoo.com>

* Update MempoolConfig field

* Test a single struct field at a time, for maintainability

Fixes #2740

* add missing options to config.toml template and docs
Refs #2232
* config#ValidateBasic
Refs #2232
* [config] timeouts as time.Duration, not ints
Why:
- native type provides better guarantees than ", in ms" comment (harder
to shoot yourself in the leg)
- flexibility: you can change units

This reverts commit d02c5d1e30.

This reverts commit 96a3502