MINOR: session: refactor alloc/lookup of sess_conns elements

MINOR: session: refactor alloc/lookup of sess_conns elements

By default backend connections are stored into idle/avail server trees.
However, if such connections cannot be shared between multiple clients,
session serves as the alternative storage.

To be able to quickly reuse a backend conn from a session, they are
indexed by their target, which is either a server or a backend proxy.
This is the purpose of 'struct sess_priv_conns' intermediary stockage
element.

Lookup and allocation of these elements are performed in several session
function, for example to add, get or remove a backend connection from a
session. The purpose of this patch is to simplify this by providing two
internal functions sess_alloc_sess_conns() and sess_get_sess_conns().

Along with this, a new BUG_ON() is added into session_unown_conn(),
which ensure that sess_priv_conns element is found when the connection
is removed from the session.