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.
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