Changes:

* Added MsgUtil.ensureEmpty, which is now called after every
  packet is received (provided it isn't dropped).
  * If the packet hasn't been fully read then this will throw
    an exception if debugging is enabled, or log an error if not.
* Added IMsgReadCtx.drop, which will log a message in debug mode,
  and also prevent the message supplier from ensuring that the
  buffer has been fully read.
* Improved debug messages a tiny bit.
