dm writecache: implement gradual cleanup

If a block is stored in the cache for too long, it will now be
written to the underlying device and cleaned up.

Add a new option "max_age" that specifies the maximum age of a block
in milliseconds.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>