lncfg+itest: expose configurable batch-commit-interval

This will permit a greater degree of tuning or customization depending
on various hardware/environmental factors.

channeldb/db.go

@@ -275,6 +275,7 @@ func CreateWithBackend(backend kvdb.Backend, modifiers ...OptionModifier) (*DB,
 	}
 	chanDB.graph = newChannelGraph(
 		chanDB, opts.RejectCacheSize, opts.ChannelCacheSize,
+		opts.BatchCommitInterval,
 	)
 
 	// Synchronize the version of database and apply migrations if needed.