Threading support #42
Description
It would be very useful to be able to multi-thread things like preloading and reification. The latter is a bit more user-driven but for preloading we could use an internal thread pool to load and parse configs.
A solution here would have to be aware of the shared reflection cache state, as well as offer a way for a config source to opt-out of threaded loading in cases where the source doesn't support multiple simultaneous reads.