Allow customizing the threadpool's parker (#264)

* Allow customizing the threadpool's parker

This patch allows the user of threadpool to customize how the worker
threads park themselves. This allows custom parking logic to be
injected. For example, this allows embedding a timer on each worker
thread.

* Call `park` instance every so often.

Since the `park` is now customizable, it might have logic that must be
called every so often. For example, a timer might have timeouts that it
must expire.

Currently, if a worker is very busy, it won't call into the `park`
instance. This patch changes this so that after every 32 task
invocations, `park` is called with a duration of zero.

tokio-executor/src/park.rs

@@ -127,6 +127,12 @@ pub trait Unpark: Sync + Send + 'static {
     fn unpark(&self);
 }
 
+impl Unpark for Box<Unpark> {
+    fn unpark(&self) {
+        (**self).unpark()
+    }
+}
+
 /// Blocks the current thread using a condition variable.
 ///
 /// Implements the [`Park`] functionality by using a condition variable. An