config: unify old and new config values for sync-freelist

In this commit, unify the old and new values for `sync-freelist`, and
also ensure that we don't break behavior for any users that're using the
_old_ value.

To do this, we first rename what was `--db.bolt.no-sync-freelist`, to
`--db.bolt.sync-freelist`. This gets rid of the negation on the config
level, and lets us override that value if the user is specifying the
legacy config option.

In the future, we'll deprecate the old config option, in favor of the
new DB scoped option.

channeldb/kvdb/config.go

@@ -12,7 +12,7 @@ const EtcdBackendName = "etcd"
 
 // BoltConfig holds bolt configuration.
 type BoltConfig struct {
-	NoFreeListSync bool `long:"nofreelistsync" description:"If true, prevents the database from syncing its freelist to disk"`
+	SyncFreelist bool `long:"nofreelistsync" description:"Whether the databases used within lnd should sync their freelist to disk. This is disabled by default resulting in improved memory performance during operation, but with an increase in startup time."`
 }
 
 // EtcdConfig holds etcd configuration.