MEDIUM: session: protect sess conns list by idle_conns_lock

MEDIUM: session: protect sess conns list by idle_conns_lock

Introduce idle_conns_lock usage to protect manipulation to <priv_conns>
session member. This represents a list of intermediary elements used to
store backend connections attached to a session to prevent their sharing
across multiple clients.

Currently, this patch is unneeded as sessions are only manipulated on a
single-thread. Indeed, contrary to idle connections stored in servers,
takeover is not implemented for connections attached to a session.
However, a future patch will introduce purging of these connections,
which is already performed for connections attached to servers. As this
can be executed by any thread, it is necessary to introduce
idle_conns_lock usage to protect their manipulation.