multi: make tower MaxUpdates configurable

This is helpful in an itest environment where we want to quickly
saturate a session.

sample-lnd.conf

@@ -1004,6 +1004,9 @@ litecoin.node=ltcd
 ; along with reduced privacy from the tower server.
 ; wtclient.session-close-range=10
 
+; The maximum number of updates to include in a tower session.
+; wtclient.max-updates=1024
+
 [healthcheck]
 
 ; The number of times we should attempt to query our chain backend before