Only create tokio runtime once for block_on

[udoprog]

Sets up a runtime in a lazy_static for blocking on current thread, and makes use of Builder::basic_scheduler to definitely avoid spinning up a thread pool for blocking the current thread.

Even if you don't want this particular PR, we should probably still make use of basic_scheduler.

[udoprog]

I was curious if this would cause any issues, and it seems like it has. I'm guessing that the postgres internals require spawning to work on a different thread, but the basic_scheduler prevents this from making progress. I've switched to using the threaded_scheduler which should work as intended if that is the case.

Note that it's still worth being explicit about which scheduler to use, which would otherwise depend on the exact set of features used!

[mehcode]

Looks fine to me minus lazy_static as I'd rather avoid the macro. But I suppose it really doesn't matter all that much until rust-lang/rust#68198 is stable.

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Regardless, thank you for noticing the TODO and fixing it. 💯