Removed unused transport/message code

More work on #228. Unencrypted envelopes are gone. In the next step, I will remove some unused code from stdafx.hpp, but I will make a separate PR for this so we can revert it easily just in case.

@haarts, @OttoAllmendinger, @0x01--:
One or all of you, please review by running some tests (by hand, or by code) that involves the transport layer. The ones I have done worked, but maybe you can think of some corner cases. Thank you.