pgsql: libpq: Add PQgetThreadLock() to mirror PQregisterThreadLock()

libpq: Add PQgetThreadLock() to mirror PQregisterThreadLock()

Allow libpq clients to retrieve the current pg_g_threadlock pointer with
PQgetThreadLock(). Single-threaded applications could already do this in
a convoluted way:

pgthreadlock_t tlock;

tlock = PQregisterThreadLock(NULL);
PQregisterThreadLock(tlock); /* re-register the callback */

/* use tlock */

But a generic library can't do that without potentially breaking
concurrent libpq connections.

The motivation for doing this now is the libpq-oauth plugin, which
currently relies on direct injection of pg_g_threadlock, and should
ideally not.

Reviewed-by: Zsolt Parragi <zsolt(dot)parragi(at)percona(dot)com>
Discussion: https://postgr.es/m/CAOYmi%2BmEU_q9sr1PMmE-4rLwFN%3DOjyndDwFZvpsMU3RNJLrM9g%40mail.gmail.com
Discussion: https://postgr.es/m/CAOYmi%2B%3DMHD%2BWKD4rsTn0v8220mYfyLGhEc5EfhmtqrAb7SmC5g%40mail.gmail.com

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/b8d76858353e40f888c809eb14a974cf13bf23ab

Modified Files
--------------
doc/src/sgml/libpq.sgml | 6 +++++-
src/interfaces/libpq/exports.txt | 1 +
src/interfaces/libpq/fe-connect.c | 7 +++++++
src/interfaces/libpq/libpq-fe.h | 10 ++++++++--
4 files changed, 21 insertions(+), 3 deletions(-)