pgsql: Use latch instead of select() in walreceiver

Use latch instead of select() in walreceiver

Replace use of poll()/select() by WaitLatchOrSocket(), which is more
portable and flexible.

Also change walreceiver to use its procLatch instead of a custom latch.

From: Petr Jelinek <petr@2ndquadrant.com>

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/597a87ccc9a6fa8af7f3cf280b1e24e41807d555

Modified Files
--------------
src/backend/postmaster/pgstat.c                    |   3 +
.../libpqwalreceiver/libpqwalreceiver.c            | 101 +++++----------------
src/backend/replication/walreceiver.c              |  18 ++--
src/backend/replication/walreceiverfuncs.c         |   6 +-
src/include/pgstat.h                               |   1 +
src/include/replication/walreceiver.h              |   3 +-
6 files changed, 43 insertions(+), 89 deletions(-)