[MEDIUM] separated nbconn into feconn and beconn

[MEDIUM] separated nbconn into feconn and beconn

The nbconn attribute in the proxies was not relevant anymore because
a frontend A may use backend B and both of them must account for their
respective connections. For this reason, there now are two separate
counters for frontend and backend connections.

The stats page has been updated to reflect the backend, but a separate
line entry for the frontend with error counts would be good.

Note that as of now, beconn may be higher than maxconn, because maxconn
applies to the frontend, while beconn may be increased due to sessions
passed from another frontend.