mds: use intrusive set for ClientLease tracking

mds: use intrusive set for ClientLease tracking

This allows to avoid additional redundant lookups in CDentry::client_leases
for some scenarios, e.g.:
* CDentry::remove_client_lease() is called from CDentry::remove_client_leases().
* CDentry::remove_client_lease() is called from Locker::remove_stale_leases()
* CDentry::remove_client_lease() is called from Locker::process_request_cap_release()

And a few similar cases.
In all of them a caller has a pointer to ClientLease object but has to
perform another lookup to remove that lease.

Signed-off-by: Garry Drankovich <garry.drankovich@clyso.com>