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>

src/makefile.linux-mingw

@@ -48,6 +48,7 @@ HEADERS = \
     main.h \
     net.h \
     noui.h \
+    protocol.h \
     rpc.h \
     script.h \
     serialize.h \
@@ -75,6 +76,7 @@ OBJS= \
     obj/keystore.o \
     obj/main.o \
     obj/net.o \
+    obj/protocol.o \
     obj/rpc.o \
     obj/script.o \
     obj/util.o \