timer: finish updating timer (#1222)

* timer: restructure feature flags
* update timer tests
* Add `async-traits` to CI

This also disables a buggy `threadpool` test. This test should be fixed in the future.

Refs #1225

tokio-timer/src/delay.rs

@@ -74,7 +74,7 @@ impl Delay {
     }
 
     // Used by `Timeout<Stream>`
-    #[cfg(feature = "timeout-stream")]
+    #[cfg(feature = "async-traits")]
     pub(crate) fn reset_timeout(&mut self) {
         self.registration.reset_timeout();
     }