Remove the separate 'stats buffer' process, letting backend stats messages

Remove the separate 'stats buffer' process, letting backend stats messages
be delivered directly to the collector process.  The extra process context
swaps required to transfer data through the buffer process seem to outweigh
any value the buffering might have.  Per recent discussion and tests.
I modified Bruce's draft patch to use poll() rather than select() where
available (this makes a noticeable difference on my system), and fixed
up the EXEC_BACKEND case.