Improved HAProxy collector

# More robust HAProxy collector with simpler config

The HAProxy collector is made more robust in the face of possible HAProxy output changes by using a proper CSV parsing library, rather than relying on indices in a list. This also makes it easier to configure, since any additional values one may wish to report to the TSD simply need to be added to a list defined at the top of the file. E.g., to add the number of requests that resulted in HTTP 5xx (server errors), one simply adds `"hrsp_5xx"` to the `METRICS_TO_REPORT` list.
This will create a new socket in every COLLECTION_INTERVAL seconds. I would suggest to create a socket in main function and use the same socket for hitting haproxy stats page.