watchtower: add forceNext to NextSessionKeyIndex

This commit adds a forceNext boolean parameter to NextSessionKeyIndex.
Setting this param to true will force the index to cycle over 1000 key
indices before returning the new key.

watchtower/wtclient/interface.go

@@ -49,8 +49,8 @@ type DB interface {
 	// (tower, blob type) pair until CreateClientSession is invoked for that
 	// tower and index, at which point a new index for that tower can be
 	// reserved. Multiple calls to this method before CreateClientSession is
-	// invoked should return the same index.
-	NextSessionKeyIndex(wtdb.TowerID, blob.Type) (uint32, error)
+	// invoked should return the same index unless forceNext is true.
+	NextSessionKeyIndex(wtdb.TowerID, blob.Type, bool) (uint32, error)
 
 	// CreateClientSession saves a newly negotiated client session to the
 	// client's database. This enables the session to be used across