Initial replay-log functionality

This patch implements a replay log with optional fs-based persistence for #725 (closed).

This does not finish the work, however, since it doesn't actually turn on the filesystem-based part; that will take integration with the introduction-point system. I've left comments describing roughly how that would look.

This is a deliberately much simpler implementation than some of the ones discussed on #725 (closed); it's internal to tor-hsservice, and so should be easy to tweak or adjust based on future needs. I'm hoping that this flexibility will let us recover from whichever absent complexity turns out to be needed in the future.

cc @Diziet who has opined on that ticket; please take the review if you want it?