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

spec/abci/abci.md

@@ -643,7 +643,7 @@ the blockchain's `AppHash` which is verified via [light client verification](../
     `Snapshot.Metadata` and/or incrementally verifying contents against `AppHash`.
     * When all chunks have been accepted, Tendermint will make an ABCI `Info` call to verify that
     `LastBlockAppHash` and `LastBlockHeight` matches the expected values, and record the
-    `AppVersion` in the node state. It then switches to fast sync or consensus and joins the
+    `AppVersion` in the node state. It then switches to block sync or consensus and joins the
     network.
     * If Tendermint is unable to retrieve the next chunk after some time (e.g. because no suitable
     peers are available), it will reject the snapshot and try a different one via `OfferSnapshot`.

spec/p2p/messages/block-sync.md

@@ -10,7 +10,7 @@ Block sync has one channel.
 
 | Name              | Number |
 |-------------------|--------|
-| BlockchainChannel | 64     |
+| BlocksyncChannel | 64     |
 
 ## Message Types