Directory cleanup could be less aggressive

[euank]

This is a followup to comments on #30 and #6.

Right now, pazi every read of the database (every z) will result in all nonexistent directories being pruned.

In reality, the pruning probably only needs to happen when I directory is actually attempted to be accessed.

Doing it less aggressively has two benefits:

1. A transient directory which comes and goes can still be zapped to when it's present so long as it wasn't zapped to specifically when it wasn't present
2. Better performance

As architected, this will not be an easy change; sorting happens at multiple layers for no good reason (and sorting is the enemy of lazyness), the frecency database doesn't clone nor implement interior mutability, etc etc.