rust-pathauditor: use interior mutability for use in multi-threaded contexts The usual recommendation for using `RwLock` or `Mutex` is that if there are about as many write as there are reads, use `Mutex`, and if there are more reads than writes, use `RwLock`. If after the main bottleneck (i.e. parallel traversal) is removed this shows up on profiles, we should investigate using the `parking_lot` since we don't need a poisoning API, or maybe move to different types of caches entirely. Differential Revision: https://phab.mercurial-scm.org/D8213