threadpool: move threadpool into tokio-executor (#1452)

The threadpool is behind a feature flag.

Refs: #1264

tokio-executor/src/threadpool/callback.rs

@@ -0,0 +1,28 @@
+use super::worker::Worker;
+
+use std::fmt;
+use std::sync::Arc;
+
+#[derive(Clone)]
+pub(crate) struct Callback {
+    f: Arc<dyn Fn(&Worker) + Send + Sync>,
+}
+
+impl Callback {
+    pub(crate) fn new<F>(f: F) -> Self
+    where
+        F: Fn(&Worker) + Send + Sync + 'static,
+    {
+        Callback { f: Arc::new(f) }
+    }
+
+    pub(crate) fn call(&self, worker: &Worker) {
+        (self.f)(worker)
+    }
+}
+
+impl fmt::Debug for Callback {
+    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(fmt, "Fn")
+    }
+}