Instead of unconditionally expiring the timer and calling a long
mt_release_contacts(), we can check if some slots are used when the
timer expires.

We can also remove the timer if we happen to receive all the releases.

The logic behind the MT_IO_FLAGS_PENDING_SLOTS could be implemented by
counting how many slots are active, but using bits feels slightly more
efficient.
Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Tested-by: Arek Burdach <arek.burdach@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>