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-threadpool/examples/depth.rs

@@ -1,3 +1,5 @@
+#![cfg(features = "broken")]
+
 extern crate env_logger;
 extern crate futures;
 extern crate tokio_threadpool;

tokio-threadpool/examples/hello.rs

@@ -1,3 +1,5 @@
+#![cfg(features = "broken")]
+
 extern crate env_logger;
 extern crate futures;
 extern crate tokio_threadpool;