CLEANUP: sessions: simplify the sess_priv_conns pool name

CLEANUP: sessions: simplify the sess_priv_conns pool name

Using "show pools detailed" on the CLI breaks the column alignment on
"sess_priv_conns" because the pool name contains spaces: "session priv
conns list", which is not welcome as pool names are truncated after the
12th chars anyway. Let's shorten it to the pool's name as done for many
other ones: sess_priv_conns.

This can be backported as far as 3.0 where this name was introduced,
because it helps when trying to sum or graph certain metrics during
debugging.