tor-persist API bug hazards

The tor-persist API leaves open the possibilities of writing code which (i) fails to call try_lock or (ii) having called try_lock, makes simultaneous updates from multiple threads/locations which are semantically incoherent due to races.

Probably: try_lock should return a lock guard. store should only be availabe on &mut guard.

(Filing this because I wrote bug (i) and this was only detected by a failing test.)