Separate network protocol from the rollback logic

[lvenerosy]

I don't know how tightly coupled they are but it would be nice if they could be isolated from each other and maybe show how to combine them in an example. For example if I use an engine which has its own protocol I might want to skip ggpo's implementation and only use the rollback.

[erebuswolf]

This pull request allows the user to inject their own network object, though there are issues with C support and it will be in limbo for a while. #42