multi: make ProofMatureDelta configurable

We add a new config option to set the `ProofMatureDelta` so the users
can tune their graphs based on their own preference over the num of
confs found in the announcement signatures.

sample-lnd.conf

@@ -1729,6 +1729,8 @@
 ; be broadcast quickly.
 ; gossip.sub-batch-delay=5s
 
+; The number of confirmations required before processing channel announcements.
+; gossip.announcement-conf=6
 
 [invoices]