Implement `tell` in MDS

I haven't been all the way through, so maybe these caps don't mean what I think they do. But in the OSDs "rw" means they can read and write the data, whereas here this is a bunch of commands that let clients do meta operations like killing each other or manipulating the MDS process. I think these should all require admin permissions.
(We can discuss making them mean something else, but I think it will confuse users.)