pgsql: Improve tests for postmaster death in auxiliary processes.

Improve tests for postmaster death in auxiliary processes.

In checkpointer and walwriter, avoid calling PostmasterIsAlive unless
WaitLatch has reported WL_POSTMASTER_DEATH.  This saves a kernel call per
iteration of the process's outer loop, which is not all that much, but a
cycle shaved is a cycle earned.  I had already removed the unconditional
PostmasterIsAlive calls in bgwriter and pgstat in previous patches, but
forgot that WL_POSTMASTER_DEATH is supposed to be treated as untrustworthy
(per comment in unix_latch.c); so adjust those two cases to match.

There are a few other places where the same idea might be applied, but only
after substantial code rearrangement, so I didn't bother.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/fd71421b0187de0e2bf76ff66b4a9433bd96c4a0

Modified Files
--------------
src/backend/postmaster/bgwriter.c     |    6 ++++--
src/backend/postmaster/checkpointer.c |   23 +++++++++++++----------
src/backend/postmaster/pgstat.c       |    9 +++++++--
src/backend/postmaster/walwriter.c    |   23 +++++++++++++----------
4 files changed, 37 insertions(+), 24 deletions(-)