Improved acceptance test that show that valid value types can be round-tripped to MongoDB.
Added acceptance test that show that valid value types can be round-tripped to MongoDB in driver-compat.  Note that CodeWithScope is not yet supported.
Moving check for correct opCode in reply message from ReplyMessage to ReplyHeader.  The advantage is that if the message is corrupt, we may detect it earlier and avoid reading the message body given a message length that is also likely corrupt.