ConcurrentObservableCollection is not thread safe

[adrianhall]

The ConcurrentObservableCollection class does not actually protect against multiple writers, so it's not thread safe - particularly the pieces that write multiple times (like AddRange).

Todo:

• Add an IAsyncLock and AsyncLock implementation.
• Add a constructor that takes an IAsyncLock as well (for testing)
• Wrap all the write operations in an IAsyncLock operation.
• Add tests to each element to ensure that the IAsyncLock actually locks during the operation.

[adrianhall]

Problem that was evident in testing is no longer present with the changes to PushAsync() locking, so closing for now.