MINOR: rhttp: add count of active conns per thread

MINOR: rhttp: add count of active conns per thread

Add a new member <nb_rhttp_conns> in thread_ctx structure. Its purpose
is to count the current number of opened reverse HTTP connections
regarding from their listeners membership.

This patch will be useful to support multi-thread for active reverse
HTTP, in order to select the less loaded thread.

Note that despite access to <nb_rhttp_conns> are only done by the
current thread, atomic operations are used. This is because once
multi-thread support will be added, external threads will also retrieve
values from others.