make some lowering operations async

[Xanewok]

Originally created by @nrc (rust-dev-tools/rls-analysis#122):

we currently wait for all lowering to be done before reporting analysis as done. However, I think we could probably report we are done before some lowering is done and only block on the main cross-referencing (e.g., #118, we could return before building the search trie).

cc #25