Remove unnecessary uses of `DashMap` and `Arc` (#3413)
## Summary
All of the resolver code is run on the main thread, so a lot of the
`Send` bounds and uses of `DashMap` and `Arc` are unnecessary. We could
also switch to using single-threaded versions of `Mutex` and `Notify` in
some places, but there isn't really a crate that provides those I would
be comfortable with using.
The `Arc` in `OnceMap` can't easily be removed because of the uv-auth
code which uses the
[reqwest-middleware](https://docs.rs/reqwest-middleware/latest/reqwest_middleware/trait.Middleware.html)
crate, that seems to adds unnecessary `Send` bounds because of
`async-trait`. We could duplicate the code and create a `OnceMapLocal`
variant, but I don't feel that's worth it.
