pgsql: Add -F option to pg_receivexlog, for specifying fsync interval.

Add -F option to pg_receivexlog, for specifying fsync interval.

This allows us to specify the maximum time to issue fsync to ensure
the received WAL file is safely flushed to disk. Without this,
pg_receivexlog always flushes WAL file only when it's closed and
which can cause WAL data to be lost at the event of a crash.

Furuya Osamu, heavily modified by me.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/3dad73e71f08abd86564d5090a58ca71740e07e0

Modified Files
--------------
doc/src/sgml/ref/pg_receivexlog.sgml | 15 +++
src/bin/pg_basebackup/pg_basebackup.c | 2 +-
src/bin/pg_basebackup/pg_receivexlog.c | 18 ++-
src/bin/pg_basebackup/receivelog.c | 227 ++++++++++++++++++++++----------
src/bin/pg_basebackup/receivelog.h | 3 +-
5 files changed, 195 insertions(+), 70 deletions(-)