pgsql: Use WaitLatch() instead of pg_usleep() at the end of backups

Use WaitLatch() instead of pg_usleep() at the end of backups

This concerns pg_stop_backup() and BASE_BACKUP, when waiting for the
WAL segments required for a backup to be archived.  This simplifies a
bit the handling of the wait event used in this code path.

Author: Bharath Rupireddy
Reviewed-by: Michael Paquier, Stephen Frost
Discussion: https://postgr.es/m/CALj2ACU4AdPCq6NLfcA-ZGwX7pPCK5FgEj-CAU0xCKzkASSy_A@mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/2aca19f2989aa938ece7306678f5494a984ece3f

Modified Files
--------------
src/backend/access/transam/xlog.c | 8 +++++---
1 file changed, 5 insertions(+), 3 deletions(-)