New tunable 'sbuf_loopcnt' to limit time spent on one socket.

New tunable 'sbuf_loopcnt' to limit time spent on one socket.

In some situations - eg SMP server, local Postgres and fast network -
pgbouncer can run recv()->send() loop many times without blocking
on either side.  But that means other connections will stall for
a long time.  To make processing more fair, limit the times
of doing recv()->send() one socket.  If count reaches limit,
just proceed processing other sockets.  The processing for
that socket will resume on next event loop.

Thanks to Alexander Schöcke for report and testing.