Expire entries and run compactions explicitly in the background

[udoprog]

Compactions are currently handled by rocksdb which spawns its own threads for this purpose.

Instead, allow the user to setup their own background process (the following is just an example):

let cleaner = cache.cleaner().entry_limit(100).interval(Duration::secs(10)).build();
tokio::spawn(cleaner);

This way, if the cleaner is dropped, so is the background procedure.