pgsql: Ignore server-side delays when enforcing wal_sender_timeout.

Ignore server-side delays when enforcing wal_sender_timeout.

Healthy clients of servers having poor I/O performance, such as
buildfarm members hamster and tern, saw unexpected timeouts.  That
disagreed with documentation.  This fix adds one gettimeofday() call
whenever ProcessRepliesIfAny() finds no client reply messages.
Back-patch to 9.4; the bug's symptom is rare and mild, and the code all
moved between 9.3 and 9.4.

Discussion: https://postgr.es/m/20180826034600.GA1105084@rfd.leadboat.com

Branch
------
REL9_4_STABLE

Details
-------
https://git.postgresql.org/pg/commitdiff/20cd88857b3a60d40cf019872cf8a5d40888e3ae

Modified Files
--------------
src/backend/replication/walsender.c | 64 +++++++++++++++++++++----------------
1 file changed, 36 insertions(+), 28 deletions(-)