Storage refactorings

[pietroalbini]

This PR includes the first batch of refactorings to the storage subsystem to better support build tests. The most notable change is we're now using a single instance of Storage across the whole application, instead of creating a new one every time we need to use it.

This doesn't contain all the changes to the storage I need to make: namely I first need to move all the configuration to Config. This PR is starting to get big though and rebasing it is a pain, so I submitted it earlier.

This is best reviewed commit-by-commit.

[jyn514]

Thanks so much! This should finally unblock the switch to tokio 🎉 and improve our response times by a lot now that we're no longer creating a new runtime for each request.

[jyn514]

Wait hold on, it looks like a runtime is still created each time start_storage_transaction is called. Can you make the runtime a field on the s3 backend instead and use OnceCell::Lazy so it's only created once?

[pietroalbini]

Addressed all review comments.