NVMe: IO queue deletion re-write

The nvme driver deletes IO queues asynchronously since this operation
may potentially take an undesirable amount of time with a large number
of queues if done serially.

The driver used to manage coordinating asynchronous deletions. This
patch simplifies that by leveraging the block layer rather than using
kthread workers and chaining more complicated callbacks.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Jens Axboe <axboe@fb.com>