config: backport the rename of fastsync to blocksync (#9259)

This is largely a cherry pick of #6755 with some additional fixups added where detected. 
This change moves the blockchain package to a package called blocksync. Additionally, it renames the relevant uses of the term `fastsync` to `blocksync`.

closes: #9227 

#### PR checklist

- [ ] Tests written/updated, or no tests needed
- [x] `CHANGELOG_PENDING.md` updated, or no changelog entry needed
- [x] Updated relevant documentation (`docs/`) and code comments, or no
      documentation updates needed

docs/tendermint-core/block-sync.md

@@ -0,0 +1,51 @@
+---
+order: 10
+---
+
+# Block Sync
+*Formerly known as Fast Sync*
+
+In a proof of work blockchain, syncing with the chain is the same
+process as staying up-to-date with the consensus: download blocks, and
+look for the one with the most total work. In proof-of-stake, the
+consensus process is more complex, as it involves rounds of
+communication between the nodes to determine what block should be
+committed next. Using this process to sync up with the blockchain from
+scratch can take a very long time. It's much faster to just download
+blocks and check the merkle tree of validators than to run the real-time
+consensus gossip protocol.
+
+## Using Block Sync
+
+To support faster syncing, Tendermint offers a `block-sync` mode, which
+is enabled by default, and can be toggled in the `config.toml` or via
+`--block_sync=false`.
+
+In this mode, the Tendermint daemon will sync hundreds of times faster
+than if it used the real-time consensus process. Once caught up, the
+daemon will switch out of Block Sync and into the normal consensus mode.
+After running for some time, the node is considered `caught up` if it
+has at least one peer and it's height is at least as high as the max
+reported peer height. See [the IsCaughtUp
+method](https://github.com/tendermint/tendermint/blob/b467515719e686e4678e6da4e102f32a491b85a0/blockchain/pool.go#L128).
+
+Note: While there have historically been multiple versions of blocksync, v0, v1, and v2, all versions
+other than v0 have been deprecated in favor of the simplest and most well understood algorithm.
+
+```toml
+#######################################################
+###       Block Sync Configuration Options          ###
+#######################################################
+[blocksync]
+
+# Block Sync version to use:
+# 
+# In v0.37, v1 and v2 of the block sync protocols were deprecated.
+# Please use v0 instead.
+#
+#   1) "v0" - the default block sync implementation
+version = "v0"
+```
+
+If we're lagging sufficiently, we should go back to block syncing, but
+this is an [open issue](https://github.com/tendermint/tendermint/issues/129).