multi: add new NoRevLogAmtData config option

In this commit, a new `--db.no-rev-log-amt-data` flag is added. The
config option is passed though to everywhere that it will be used. Note
that it is still a no-op in this commit. An upcoming commit will make
use of the flag.

lncfg/db.go

@@ -85,6 +85,8 @@ type DB struct {
 	NoGraphCache bool `long:"no-graph-cache" description:"Don't use the in-memory graph cache for path finding. Much slower but uses less RAM. Can only be used with a bolt database backend."`
 
 	PruneRevocation bool `long:"prune-revocation" description:"Run the optional migration that prunes the revocation logs to save disk space."`
+
+	NoRevLogAmtData bool `long:"no-rev-log-amt-data" description:"If set, the to-local and to-remote output amounts of revoked commitment transactions will not be stored in the revocation log. Note that once this data is lost, a watchtower client will not be able to back up the revoked state."`
 }
 
 // DefaultDB creates and returns a new default DB config.