lorri daemon: stop watching a project’s files after a timeout

[Profpatsch]

This [the lorri daemon] is done now, right?

Everything except the timeout part, which is why I haven’t closed this issue yet.

Originally posted by @Profpatsch in #65 (comment)

If the daemon runs for a long time and watches a lot of projects, it might run out of available watch handles to watch more files; this is essentially a resource leak.

We should introduce a timeout that automatically drops files from the watcher after a certain amount of time has passed without any new requests for evaluation of a nix file.

[nyarly]

Related, in #365 there's a comment-in-passing about lorri stop, which might just be "expire this project immediately" (maybe the command should be lorri expire to make that clear).