This commit is an attempt at detecting unconsumed data on a socket when
we pull it from the connection pool.

Two new INI settings are introduced related to the changes:

redis.pconnect.pool_detect_dirty:

Value Explanation
----- ----------------------------------------------------------------
    0 Don't execute new logic at all.
    1 Abort and close the socket if we find unconsumed bytes in the
      read buffer.
    2 Seek to the end of our read buffer if we find unconsumed bytes
      and then poll the socket FD to detect if we're still readable
      in which case we fail and close the socket.

redis.pconnect.pool_poll_timeout:

The poll timeout to employ when checking if the socket is readable.
This value is in milliseconds and can be zero.

Review changes

See #2013

Perform cheaper PHP liveness check first.

See #2013