Add new STATS worker message.

This implements phase 1 of monitoring by basically checking that
HAProxy is alive. A new STATS message is recognized. The response
to this message will, so far, contain only a PASS/FAIL response,
and an error message in the case of FAIL.

The next phase will be to add backend node checking. This will
likely be combined with reporting usage statistics at a later phase.

Change-Id: Ie381c4cbb91dad90074f5addd3c4e3cf24dd33d8