Update haproxy-sessions-by-servers

Updated haproxy-sessions-by-servers to look at the current sessions by individual servers.  I believe this was the original intent of the script (it was broken by default when symlinking it in /etc/munin/plugins/ (/etc/munin/plugins/haproxy-sessions-by-servers).

NOW SUPPORTS:
- URL + Socket Stats
- Dynamic list of individual backend member servers
- tested / works with munin-node

NUANCES:
- may be annoying with an absurd number of backend member nodes (I have 10 on this particular haproxy instance)