runtime: cleanup and add config options (#1807)

* runtime: cleanup and add config options

This patch finishes the cleanup as part of the transition to Tokio 0.2.
A number of changes were made to take advantage of having all Tokio
types in a single crate. Also, fixes using Tokio types from
`spawn_blocking`.

* Many threads, one resource driver

Previously, in the threaded scheduler, a resource driver (mio::Poll /
timer combo) was created per thread. This was more or less fine, except
it required balancing across the available drivers. When using a
resource driver from **outside** of the thread pool, balancing is
tricky. The change was original done to avoid having a dedicated driver
thread.

Now, instead of creating many resource drivers, a single resource driver
is used. Each scheduler thread will attempt to "lock" the resource
driver before parking on it. If the resource driver is already locked,
the thread uses a condition variable to park. Contention should remain
low as, under load, the scheduler avoids using the drivers.

* Add configuration options to enable I/O / time

New configuration options are added to `runtime::Builder` to allow
enabling I/O and time drivers on a runtime instance basis. This is
useful when wanting to create lightweight runtime instances to execute
compute only tasks.

* Bug fixes

The condition variable parker is updated to the same algorithm used in
`std`. This is motivated by some potential deadlock cases discovered by
`loom`.

The basic scheduler is fixed to fairly schedule tasks. `push_front` was
accidentally used instead of `push_back`.

I/O, time, and spawning now work from within `spawn_blocking` closures.

* Misc cleanup

The threaded scheduler is no longer generic over `P :Park`. Instead, it
is hard coded to a specific parker. Tests, including loom tests, are
updated to use `Runtime` directly. This provides greater coverage.

The `blocking` module is moved back into `runtime` as all usage is
within `runtime` itself.

tokio-test/src/io.rs

@@ -1,3 +1,5 @@
+#![cfg(not(loom))]
+
 //! A mock type implementing [`AsyncRead`] and [`AsyncWrite`].
 //!
 //!

tokio-test/src/lib.rs

@@ -14,6 +14,7 @@
 //! Tokio and Futures based testing utilites
 
 pub mod io;
+
 mod macros;
 pub mod task;
 
@@ -27,7 +28,11 @@ pub mod task;
 pub fn block_on<F: std::future::Future>(future: F) -> F::Output {
     use tokio::runtime;
 
-    let mut rt = runtime::Builder::new().basic_scheduler().build().unwrap();
+    let mut rt = runtime::Builder::new()
+        .basic_scheduler()
+        .enable_all()
+        .build()
+        .unwrap();
 
     rt.block_on(future)
 }