dm thin: defer whole cells rather than individual bios

This avoids dropping the cell, so increases the probability that other
bios will collect within the cell, rather than being passed individually
to the worker.

Also add required process_cell and process_discard_cell error handling
wrappers and set associated pool-mode function pointers accordingly.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>