Ceph » Messengers

I made a start on this with my interface work, but there are still
1) a lot of public functions that ought to be private or protected.
2) Functions needing documentation.
3) Subclasses that need to be reordered, have more implementation pushed down into the C file, etc.

Basically, the messenger is an important part of our system. Its header should be legible to outside contributors so they can do things like implement their own on a different network fabric; so newcomers can understand the interface in a reasonable amount of time, etc.