Vector: Remove tokio-compat

[fanatid]

3449 remove tokio:0.1, but we still have tokio-compat because use futures:0.1 in a lot of places.

~Before remove tokio-compat I propose update all tests to crate::test_util::test macros (instead manually created Runtime in each test). Macros added in #3474.~

Plan:

• [x] Replace runtime().block_on* to #[tokio::test]. #3505 #3626
• [x] Investigate why some test require threaded_scheduler and/or core_threads = 2. #3578 #3596
• [x] Update benches #3626
• [x] Remove tokio-compat #3627

[MOZGIII]

Is the plan to also remove our Runtime abstraction along with the tokio-compat? The reason I'm asking is I think it may be useful even after tokio upgrade - however it's not totally necessary so we might want to cut it for simplicity...

[lukesteensen]

Is the plan to also remove our Runtime abstraction along with the tokio-compat?

My assumption is that we will be using Runtime in extremely few places as we move off tokio-compat and onto #[tokio::test]. That makes it much less important and I'd be in favor of just removing it unless there is a good reason to keep it.