[MINOR] acl: add fe_conn, be_conn, queue, avg_queue

[MINOR] acl: add fe_conn, be_conn, queue, avg_queue

These ACLs are used to check the number of active connections on the
frontend, backend or in a backend's queue. The avg_queue returns the
average number of queued connections per server, and for this, divides
the total number of queued connections by the number of alive servers.

The dst_conn ACL has been slightly changed to more reflect its name and
original usage, which is to return the number of connections on the
destination address/port (the socket) and not the whole frontend.