Implement a dirmgr::storage replacement for no-sqlite environments like wasm

If we want to run in a wasm-style world someday, we'll need a different solution to dirmgr caching. The current sqlite-based implementation is good for most systems, but we'll need something different for environments that don't have a working sqlite port with working persistence.

The first step here is to make a Storage trait for the tor-dirmgr crate.