More deprecation notices (relates #2036)

[udoprog]

tokio-buf and tokio-current-thread appear to have straight up been deprecated. It would be nice to add a notice on what to use instead for current thread (futures::executor::block_on is an alternative), but I'm not sure what to do.

Using the basic scheduler works, but is somewhat awkard:

let mut rt = tokio::runtime::Builder::new().basic_scheduler().enable_all().build()?;
let set = tokio::task::LocalSet::new();
let result = set.block_on(&mut rt, /* future */);

[LucioFranco]

this is fine no one was really using it