Added locking logic to AzureMessageQueueReceiver.GetMessage to resolve failure to get messages from queue.

Due to concurrent access and the lack of thread-safety, AzureMessageQueueReceiver.GetMessage is, on occasion, failing to read messages from the input queue. There aren't any exceptions, just no messages processed after queue.BeginGetMessages completes, even though there are messages in the input queue. With the addition of the locking logic, the issue appears to be resolved.