Start moving protocol-specific code to protocol.[ch]pp

Move CMessageHeader from net.h to protocol.[ch]pp, with the
implementation in the .cpp compilation unit (compiling once is enough).

This commit does *not* and should not modify *any* code, it only moves
it from net.h and splits it across protocol.cpp and protocol.hpp.

Indentation changes aside the closest thing to a modification of code is
the addition of the 'TODO' comment (the execution of which requires code
modifications and thus doesn't belong in this commit).

Signed-off-by: Giel van Schijndel <me@mortis.eu>