PATCH: add pg_current_xlog_flush_location function

Hi,

attached is a patch adding a function pg_current_xlog_flush_location(),
which proved quite useful when investigating the ext4 data loss bug.
It's mostly what was already sent to that thread, except for docs that
were missing in the initial version.

I'll put this into 2016-01 CF.

regards

--
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services