Prevent GCD deadlocks by comparing dispatch_get_current_queue() with _queue

Check for dispatch queue deadlocks by comparing current queue with `_queue` object.

This approach:

* doesn't require an additional constant
* works on OS X 10.6 SDK (which doesn't have `dispatch_queue_set_specific()` / `dispatch_queue_get_specific()`)

Please note that `dispatch_get_current_queue()` is deprecated on OS X 10.9 though:

```
src/fmdb/FMDatabaseQueue.m:134:12: 'dispatch_get_current_queue' is deprecated: first deprecated in
OS X 10.9
```