multi: make SubBatchDelay configurable

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

sample-lnd.conf

@@ -1423,6 +1423,10 @@ litecoin.node=ltcd
 ; gossip.max-channel-update-burst=10
 ; gossip.channel-update-interval=1m
 
+; The duration to wait before sending the next announcement batch if there are
+; multiple. Use a small value if there are a lot announcements and they need to
+; be broadcast quickly.
+; gossip.sub-batch-delay=5s
 
 [invoices]