watchtower: extend client databse with CRUD operations for towers

These operations are currently unused, but will be integrated into the
TowerClient at a later point as future preparation for the
WatchtowerClient RPC subserver, which will allow users to add, remove,
and list the watchtowers currntly in use.

watchtower/wtdb/client_session.go

@@ -18,6 +18,10 @@ const (
 	// CSessionActive indicates that the ClientSession is active and can be
 	// used for backups.
 	CSessionActive CSessionStatus = 0
+
+	// CSessionInactive indicates that the ClientSession is inactive and
+	// cannot be used for backups.
+	CSessionInactive CSessionStatus = 1
 )
 
 // ClientSession encapsulates a SessionInfo returned from a successful