Enable multithread for crate download/verify

[hehaoqian]

This should significantly increase the download speed for crates.

Previously, It took days to download all crates
Now, it takes hours (tested with 64 threads)

[hehaoqian]

@drmikehenry Modify this PR directly if you have found any problems. Thanks

[drmikehenry]

Thanks for your pull request. I'd like to take a little time to mull it over and do some experimentation.

[drmikehenry]

Thanks for the suggestion to improve performance via parallel downloading. After some research, I have no confidence that the requests library is thread-safe (see, for example, the unclosed ticket psf/requests#2766 which merely asks for documentation to answer the question whether requests is thread-safe; this question has remained apparently unanswerable for nearly six years). Rather than venture down the multi-threading route, I've taken this as an opportunity to switch from requests to httpx and to implement asynchronous parallel downloading via the trio library. Feel free to open an issue if you discover any problems with this new implementation.