einhorn plugins have no way to persist state across reexecs

When einhorn does a stateful reexec, it passes the contents of `Einhorn::State` back to itself over a self-pipe. But plugins don't have any easy ability to mimic that behavior.

I think we should give them some hook to pass state. The easy solution is probably to let them sneak attributes onto `Einhorn::State` itself, but we should probably come up with something cleaner.