Ensure stream position to be reset before returning task

[ali-ince]

When handling ReadNextMessagesAsync calls, we record message stream buffer position and reset it to that recorded value just after actual read messages operation completes. However, the returned task from ReadNextMessagesAsync call does not include the task continuation that resets the position - and returns the original task. This may sometimes cause the message reading operation resume from an incorrect buffer position which causes an unexpected byte sequence and halts with ProtocolException.

This PR fixes the issue by returning the expeccted position resetting task continuation as the result from ReadNextMessagesAsync method.