engines/rados: changed polling to completion callbacks methodology

engines/rados: changed polling to completion callbacks methodology

Previously, getevent() function polled operations that have been submitted to check if some of them have already finished.
Now, each rados operation invokes completion callback. This callback adds to list of completed operations.
In this version getevent() only peeks if completed list has some operations.

There are two benefits:
1) small one - this works faster then previous version
2) big one - when there is a huge amount of operations in fly, getevent() might be more overloaded then producer. In this scenario some old operations are finished, but not picked up and properly noticed. This caused absurdely long execution times, when actually everything was working properly.

Signed-off-by: Adam Kupczyk <akupczyk@redhat.com>