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

cmd/tendermint/commands/root.go

@@ -53,6 +53,11 @@ func ParseConfig(cmd *cobra.Command) (*cfg.Config, error) {
 	if err := conf.ValidateBasic(); err != nil {
 		return nil, fmt.Errorf("error in config file: %v", err)
 	}
+	if warnings := conf.CheckDeprecated(); len(warnings) > 0 {
+		for _, warning := range warnings {
+			logger.Info("deprecated usage found in configuration file", "usage", warning)
+		}
+	}
 	return conf, nil
 }
 

cmd/tendermint/commands/run_node.go

@@ -31,7 +31,7 @@ func AddNodeFlags(cmd *cobra.Command) {
 		"socket address to listen on for connections from external priv_validator process")
 
 	// node flags
-	cmd.Flags().Bool("fast_sync", config.FastSyncMode, "fast blockchain syncing")
+	cmd.Flags().Bool("block_sync", config.BlockSyncMode, "sync the block chain using the blocksync algorithm")
 	cmd.Flags().BytesHexVar(
 		&genesisHash,
 		"genesis_hash",