Reduce idle power consumption of stats collector process.

Latch-ify the stats collector, so that it does not need an arbitrary wakeup
cycle to check for postmaster death.  The incremental savings in idle power
is pretty marginal, since we only had it waking every two seconds; but I
believe that this patch may also improve the collector's performance under
load, by reducing the number of kernel calls made per message when messages
are arriving constantly (we now avoid a select/poll call except when we
need to sleep).  The change also reduces the time needed for a normal
database shutdown on platforms where signals don't interrupt select().