pgsql: Fix bug in clean shutdown of walsender that pg_receiving is conn

Fix bug in clean shutdown of walsender that pg_receiving is connecting to.

On clean shutdown, walsender waits for all WAL to be replicated to a standby,
and exits. It determined whether that replication had been completed by
checking whether its sent location had been equal to a standby's flush
location. Unfortunately this condition never becomes true when the standby
such as pg_receivexlog which always returns an invalid flush location is
connecting to walsender, and then walsender waits forever.

This commit changes walsender so that it just checks a standby's write
location if a flush location is invalid.

Back-patch to 9.1 where enough infrastructure for this exists.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/65e8dbb1869db542c93333fc809f34c7d3f7d9bd

Modified Files
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src/backend/replication/walsender.c | 13 ++++++++++++-
1 file changed, 12 insertions(+), 1 deletion(-)