Endeavor to make pgstats buffer process (a) safe and (b) useful.

Endeavor to make pgstats buffer process (a) safe and (b) useful.
Make sure it exits immediately when collector process dies --- in old code,
buffer process would hang around and compete with the new buffer process
for packets.  Make sure it doesn't block on writing the pipe when the
collector falls more than a pipeload behind.  Avoid leaking pgstats FDs
into every backend.