dm vdo slab-depot: read refcount blocks in large chunks at load time

At startup, vdo loads all the reference count data before the device
reports that it is ready. Using a pool of large metadata vios can
improve the startup speed of vdo. The pool of large vios is released
after the device is ready.

During normal operation, reference counts are updated 4kB at a time,
as before.

Signed-off-by: Ken Raeburn <raeburn@redhat.com>
Signed-off-by: Matthew Sakai <msakai@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>