Let MTPDocumentsProvider load documents in background thread.

The CL introduce DocumentLoader class that has a responsibility for loading
documents on background thread. When the provider is requested documents and the
result is not cached, it returns the cursor containing the first 10 items with
extra loading flag. Then it loads the rest of documents into cache on background
thread and notifies update regularly.

BUG=23067619

Change-Id: I7ac9d919bc3ffee1960ae3e8e7272a792a982ea8