gm: remove some unimplemented/unused declarations
gm: remove virtual function table

Pretty sure we're not going to be having any baby group management
classes anytime soon, or ever. And if so, I'd vote to have a virtual
destructor first.
gm: should not have any protected anything

gm is not designed for inheritance, so it doesn't make sense to have
protected functions, and it definitely doesn't make sense to have
protected data members.
gm: remove most of the public interface

For some reason, GroupManagement decided that its implementation details
ought to be part of its public interface. This looks clearly wrong.
gm: remove global using declarations

If a using declaration appears in a header file, it needs to be scoped
to a namespace or a class, rather than pollute the rest of the project.

But it turns out that all of these declarations are unused. Goodbye.
gm: remove unnecessary includes from header

Turns out a couple of modules accidentally dependend on pollution from
CGlobalPeerList.hpp, so add that where it's actually required.
gm: remove unused ready message

This is never sent and never handled.
gm: inherit privately from IPeerNode

I strongly believe that we should not be inheriting from IPeerNode at
all, but inheriting privately is a quick fix that makes this
non-dangerous.
Merge pull request #334 from FREEDM-DGI/gm

Gm