Description of problem:
Due to a race condition, Live Merge execution can claim successful completion after only removing the snapshot record from the database, skipping the merge operation altogether.

Version-Release number of selected component (if applicable):
Witnessed on ovirt master commit 01b8febd9b; however looking through the logs it appears this is a possibility in all versions with this feature.

How reproducible:
Based on some testing on my work laptop:
 - before 3.6.0, approximately 1-in-5000 executions
 - 3.6.0 or later, approximately 1-in-500 executions
(See below for more detail.)

Steps to Reproduce:
1. Execute live merge
2. Hope for the best
3. 
The best way to reproduce the bug is to make a small code change that makes reproduction much more likely.  Changing the loop initialization In CommandExecutor to use TimeUnit.MILLISECONDS instead of TimeUnit.SECONDS allows reproduction after just a handful of attempts.

Actual results:
RemoveSnapshotCommand executes without RemoveSnapshotSingleDiskLiveCommand.  This means:
- the snapshot record is removed from the db with no relevant updates
- the image record remains in the db with no relevant updates
- no vdsm verbs are called, thus the merge is not complete
- data loss after subsequent merges??

Expected results:
- db fields should be updated and removed as appropriate
- the merge should occur

Additional info:

The root cause of this is a race caused by the RemoveSnapshotCommandCallback.doPolling() method not accounting for the NOT_STARTED task status of its RemoveSnapshotSingleDiskLiveCommand children.  When RemoveSnapshotCommand starts its children, the children are executed asynchronously.  Usually (in my tests) the children start before the parent is complete; however, sometimes the order is reversed and this opens up a small window of time during which the callback might execute and find the child commands in the NOT_STARTED state.

When this occurs, the callback errantly considers the children to have completed successfully, causing the end-action callbacks on the parent and child commands to be triggered.  The RemoveSnapshotSingleDiskCommand is thus bypassed, causing the merge to not complete.

Testing on my laptop gives evidence that the behavior is usually what is desired.  8 of 10 tests shows a favorable command sequence, whereas 2 tests showed an small (8ms) window of time where a well-timed interruption would trigger this bug.  The probabilities listed above are based on this small sample.