Fix start WAL filename for concurrent backups from standby

On a standby, ThisTimelineID is always 0, so we would generate a
filename in timeline 0 even for other timelines. Instead, use starttli
which we have retreived from the controlfile.

Report by: Francesco Canovai in bug #14230
Author: Marco Nenciarini
Reviewed by: Michael Paquier and Amit Kapila