multi: make PendingCommitInterval configurable

sample-lnd.conf

@@ -297,6 +297,13 @@
 ; capped at 1 hour. The default is 50 milliseconds.
 ; channel-commit-interval=1h
 
+; The maximum time that is allowed to pass while waiting for the remote party
+; to revoke a locally initiated commitment state. Setting this to a longer
+; duration if a slow response is expected from the remote party or large
+; number of payments are attempted at the same time. 
+; (default: 1m0s)
+; pending-commit-interval=5m
+
 ; The maximum number of channel state updates that is accumulated before signing
 ; a new commitment.
 ; channel-commit-batch-size=10