Remove reference counting from MessageData

[ajsutton]

PR description

Modifies MessageData so that it's backed by a BytesValue instance instead of a netty ByteBuf. This provides a number of benefits:

• Removes the need to use reference counting on all MessageData instances thus eliminating an entire category of memory allocation exhaustion bugs. These generated errors like io.netty.util.internal.OutOfDirectMemoryError: failed to allocate 16777216 byte(s) of direct memory (used: 1828716583, max: 1836580864)
• BytesValue is a simpler interface and immutable so we can significantly reduce the amount of data copying that's required. Previously we copied every incoming message at least 3 times, now it's only a single copy from the netty ByteBuf to a byte[] the BytesValue wraps.
• Simplifies quite a lot of code by avoiding the need to convert between ByteBuf and ByteValues and removing all the try{} finally {message.release()} blocks.