runtime: rename current_thread -> basic_scheduler (#1769)

It no longer supports executing !Send futures. The use case for
It is wanting a “light” runtime. There will be “local” task execution
using a different strategy coming later.

This patch also renames `thread_pool` -> `threaded_scheduler`, but
only in public APIs for now.

tokio-test/src/lib.rs

@@ -27,7 +27,7 @@ pub mod task;
 pub fn block_on<F: std::future::Future>(future: F) -> F::Output {
     use tokio::runtime;
 
-    let mut rt = runtime::Builder::new().current_thread().build().unwrap();
+    let mut rt = runtime::Builder::new().basic_scheduler().build().unwrap();
 
     rt.block_on(future)
 }