Replace async_once with tokio::sync::OnceCell

[Expyron]

async_once is unmaintained and has been reported unsound.

tokio's parking_lot feature has been added in order to have access to OnceCell::const_new(). This is only necessary in older tokio versions.

[abr-egn]

Thank you for contributing this! I've authorized an evergreen run and assuming that passes I'll merge this in.

[abr-egn]

It looks like this doesn't compile with the sync feature enabled, and also needs a pass through rustfmt +nightly --unstable-features. Once those are fixed I can merge this :)

[Expyron]

I pushed an update:

• Ran rustfmt
• I'm not sure what happened with the sync feature, but I don't think it was the changes in that PR. I tried to fix it by updating the log dependency, which was what caused an issue on my machine. If it fails again, can you share the error message?

[abr-egn]

Sorry, forgot that evergreen runs aren't publicly visible! Running cargo check --tests --features tokio-sync will reproduce the errors:

error[E0618]: expected function, found `get_client_options`
  --> src/sync/test.rs:64:19
   |
39 | / lazy_static! {
40 | |     static ref get_client_options: ClientOptions =
41 | |         runtime::block_on(async { crate::test::get_client_options().await.clone() });
42 | | }
   | |_- `sync::test::get_client_options` defined here
...
64 |       let options = get_client_options().clone();
   |                     ^^^^^^^^^^^^^^^^^^--
   |                     |
   |                     call expression requires function

... etc ...

[Expyron]

Pushed an update fixing the sync tests

[abr-egn]

LGTM! Tagging in Isabel.

[isabelatkinson]

lgtm, thanks for your contribution!