Test FDBDirectory consistency while cross-transaction merges are ongoing

[wshaib-apple]

1. Lock a directory, and commit, then try to update a record, or save a record, or do a search, and assert that nothing is corrupted (or that the user request fails)
2. A test of what lucene does when a merge loses its lock
3. A chaos test of two threads, one constantly trying to merge, and one trying to update a record, or save a new record or do a search. At the end the index should be validated for consistency. It is expected (for now) that user operations will fail.
4. A test where there are multiple threads both trying to do merges. At the end the index should be validated for consistency.