Start WCAR replay at a specific GTID

As a user id like to start WCAR replay at a specific GTID so that I dont need a replay capture to perfectly align with a backup to restore so that the test is consistent.
By allowing to specify the GTID to start the replay, i can take a backup whenever after the start capture is begun and have the replay start wherever the backup was taken.

Furthermore this enabled isolating the test to when a specific bug occurs say @ x GTID. to begin shortly before. Optionally taking a timestamp to begin the replay would be good, as well as an auto mode to start whever the GTID on the server currently is.