[MEDIUM] session: add concurrent connections counter

[MEDIUM] session: add concurrent connections counter

The new "conn_cur" session counter has been added. It is automatically
updated upon "track XXX" directives, and the entry is touched at the
moment we increment the value so that we don't consider further counter
updates as real updates, otherwise we would end up updating upon completion,
which may not be desired. Probably that some other event counters (eg: HTTP
requests) will have to be updated upon each event though.

This counter can be matched against current session's source address using
the "src_conn_cur" ACL.