bg task does not release the mutex on its own

```
2018-04-05T21:50:36 [BG] [Trace] [Signal_Windows.Lib.LibUtils] System lock releasing, sync context = (null)
2018-04-05T21:50:36 [BG] [Warning] [Signal_Windows.Lib.LibUtils] System lock failed to unlock! Object synchronization method was called from an unsynchronized block of code.
   at System.Threading.Mutex.ReleaseMutex()
   at Signal_Windows.Lib.LibUtils.Unlock()
```