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
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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(-)