MINOR: session: rename private conns elements

MINOR: session: rename private conns elements

By default, backend connections are attached to a server instance. This
allows to implement connection reuse. However, in some particular cases,
connection cannot be shared accross several clients. These connections
are considered and private and are attached to the session instance
instead.

These private connections are also indexed by the target server to not
mix them. All of this is implemented via a dedicated structure
previously named struct sess_srv_list.

Rename it to better reflect its usage to struct sess_priv_conns. Also
rename its internal members and all of the associated functions.

This commit is only a renaming, thus no functional impact is expected.