librbd: parallelize rollback

Use a SimpleThrottle like trim_image() to limit the number of
requests in flight.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
librbd: only send non-zero copyup data

If the parent image is logically zero for the range of a child object,
it's equivalent to the object not existing. Save some I/O and network
bandwidth and don't send the useless zeroes.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
librbd: parallelize and simplify flatten

Flattening reads the logical child object from the parent image, and
then does a copyup operation if the data is non-zero. This is
equivalent to doing a zero-length write to each object in the
child image. Do this instead, so that we can easily control how
many are in flight, and eliminate some code as well.

Since we no longer read from the parent within the flatten function,
the buffer is not needed. It would be leaked in some error conditions,
but since's it's unecessary we can just get rid of it.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
librbd: move completion release into rbd_ctx_cb()

All the users of rbd_ctx_cb() do this separately right now, but
there's no reason to keep the completion around after the nested
completion has been called. Also declare rbd_ctx_cb() in the header
so it can be used before its definition.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
librbd: run copy in parallel

Instead of using read_iterate(), loop over each period of objects in
the source, read from them asynchronously, and then asynchronously
write to the destination.

The callbacks make this a bit more complex, but it can perform much
better.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>
Throttle: move start_op() to C_SimpleThrottle constructor

This is done by all callers right before constructing this.
Since C_SimpleThrottle is already responsible for calling ->end_op(),
it makes sense to call start_op() there too.

Signed-off-by: Josh Durgin <josh.durgin@inktank.com>