multi: make tower MaxUpdates configurable

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

lncfg/wtclient.go

@@ -22,6 +22,10 @@ type WtClient struct {
 	// of blocks to wait after the last channel of a session is closed
 	// before sending the DeleteSession message to the tower server.
 	SessionCloseRange uint32 `long:"session-close-range" description:"The range over which to choose a random number of blocks to wait after the last channel of a session is closed before sending the DeleteSession message to the tower server. Set to 1 for no delay."`
+
+	// MaxUpdates is the maximum number of updates to be backed up in a
+	// single tower sessions.
+	MaxUpdates uint16 `long:"max-updates" description:"The maximum number of updates to be backed up in a single session."`
 }
 
 // Validate ensures the user has provided a valid configuration.