Add locking to focused merging to prevent conflicts with supervoxel splitting

[hubbardp]

The Neu3 cleaving tool uses the Librarian to check out (lock) the body being cleaved. All current work on supervoxel splitting is being done through the cleaving tool, so the bodies of all supervoxels being split are being locked.

So the Neu3 focused merging tool should also use the Librarian to lock the bodies mapped from the two supervoxel in the merging task.

Doing so should prevent an odd problem that occurred today, which may be representative of other odd problems that are occurring recently. A user in the middle of a focused merging session reported that only one body was appearing for a particular task. The problem was that one of the supervoxels in that task no longer mapped to a body. It turns out that the supervoxel had been split, so its ID is no longer in use. The Neu3 code for the focused merging task checks for this situation, and skips the task if the mapping fails, but there is nothing to prevent the supervoxel from being split after the merging task has been started. Based on the logs, that is what happened in this case.