Fixes and session monitoring for check-haproxy

Requiring the haproxy gem (even via silent_require) was exiting without running the check. Perhaps a better solution would be to fix the haproxy gem, but for now I've modified this check to do the reading and parsing of the HAProxy stat socket.

Adding support for status="OPEN" to be included in the UP count, which is what HAProxy 1.4.18 is using for FRONTEND services.

Add monitoring of active sessions (with respect to session limits) for each service via -C and -W options. This brings this check up to parity with a Nagios check we were previously using.