multi: make SubBatchDelay configurable

This commit adds a new config option `--gossip.sub-batch-delay` so we
can speed up our itest.

server.go

@@ -1013,7 +1013,7 @@ func newServer(cfg *Config, listenAddrs []net.Addr,
 		HistoricalSyncTicker:    ticker.New(cfg.HistoricalSyncInterval),
 		NumActiveSyncers:        cfg.NumGraphSyncPeers,
 		MinimumBatchSize:        10,
-		SubBatchDelay:           time.Second * 5,
+		SubBatchDelay:           cfg.Gossip.SubBatchDelay,
 		IgnoreHistoricalFilters: cfg.IgnoreHistoricalGossipFilters,
 		PinnedSyncers:           cfg.Gossip.PinnedSyncers,
 		MaxChannelUpdateBurst:   cfg.Gossip.MaxChannelUpdateBurst,