MINOR: server: shard by thread sess_conns member

MINOR: server: shard by thread sess_conns member

Server member <sess_conns> is a mt_list which contains every backend
connections attached to a session which targets this server. These
connecions are not present in idle server trees.

The main utility of this list is to be able to cleanup these connections
prior to removing a server via "del server" CLI. However, this procedure
will be adjusted by a future patch. As such, <sess_conns> member must be
moved into srv_per_thread struct. Effectively, this duplicates a list
for every threads.

This commit does not introduce functional change. Its goal is to ensure
that these connections are now ordered by their owning thread, which
will allow to implement a purge, similarly to idle connections attached
to servers.