watchtower: add GetClientSession func to DB

This commit adds a new `GetClientSession` method to the tower client DB
which can be used to fetch a session by its ID from the DB.

watchtower/wtclient/interface.go

@@ -64,6 +64,11 @@ type DB interface {
 		...wtdb.ClientSessionListOption) (
 		map[wtdb.SessionID]*wtdb.ClientSession, error)
 
+	// GetClientSession loads the ClientSession with the given ID from the
+	// DB.
+	GetClientSession(wtdb.SessionID,
+		...wtdb.ClientSessionListOption) (*wtdb.ClientSession, error)
+
 	// FetchSessionCommittedUpdates retrieves the current set of un-acked
 	// updates of the given session.
 	FetchSessionCommittedUpdates(id *wtdb.SessionID) (