Code review comment for lp:~mordred/drizzle/plugin-slot-reorg

I've made changes based on Brian's comments. What were the slot/service methods are now static methods on the plugin classes themselves. This did allow us to achieve a very nice code cleanup in drizzled/plugin/registry.h - which is still compile-time safe, so I'm happy about that.

After this branch, I have a style-cleanup branch that builds on this one, and then a branch which adds a Plugin base class to all plugin types.