client: drop unused release_lease()

Signed-off-by: Sage Weil <sage@inktank.com>
client: use session->inst where possible

Removing remaining lookups up the mds inst in the MDSMap.  Instead, use
the MetaSession inst field.

Signed-off-by: Sage Weil <sage@inktank.com>
client: use Connection* in MetaSession instead of the inst/addr

This moves us to a cleaner API, and makes the session lifetime explicit
and clean.  Yay!

Signed-off-by: Sage Weil <sage@inktank.com>