librbd: use task finisher thread for image open/close callbacks

librbd: use task finisher thread for image open/close callbacks

There was a potential race condition with utilizing the AsioEngine
to deliver asynchronous image open and close callbacks. This left
the potential for the io_context thread to attempt to destroy itself.

This commit changes the behavior of the image open and close callbacks
to always delete the ImageCtx (now matches the synchronous API behavior)
and it always invokes the callback in Finisher thread whose lifetime is
tied to the CephContext.

Signed-off-by: Jason Dillaman <dillaman@redhat.com>