watchtower/wtclient/interface+wtmock: add session key reservation

watchtower/wtclient/interface.go

@@ -23,6 +23,14 @@ type DB interface {
 	// LoadTower retrieves a tower by its tower ID.
 	LoadTower(uint64) (*wtdb.Tower, error)
 
+	// NextSessionKeyIndex reserves a new session key derivation index for a
+	// particular tower id. The index is reserved for that tower 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(uint64) (uint32, error)
+
 	// CreateClientSession saves a newly negotiated client session to the
 	// client's database. This enables the session to be used across
 	// restarts.