pgsql: Make walsender more responsive.

Make walsender more responsive.

Per testing by Andres Freund, this improves replication performance
and reduces replication latency and latency jitter.  I was a bit
concerned about moving more work into XLogInsert, but testing seems
to show that it's not a problem in practice.

Along the way, improve comments for WaitLatchOrSocket.

Andres Freund.  Review and stylistic cleanup by me.

Branch
------
master

Details
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http://git.postgresql.org/pg/commitdiff/f83b59997d29f06c3d67e7eb9a1f2c9cd017d665

Modified Files
--------------
src/backend/access/transam/twophase.c |   21 ---------------------
src/backend/access/transam/xact.c     |    7 -------
src/backend/access/transam/xlog.c     |   25 ++++++++++++++++++-------
src/backend/port/unix_latch.c         |    3 +++
src/backend/port/win32_latch.c        |    4 ++++
src/backend/replication/walsender.c   |   11 ++++++++++-
src/include/replication/walsender.h   |   24 ++++++++++++++++++++++++
7 files changed, 59 insertions(+), 36 deletions(-)