sync: move into tokio crate (#1705)

A step towards collapsing Tokio sub crates into a single `tokio`
crate (#1318).

The sync implementation is now provided by the main `tokio` crate.
Functionality can be opted out of by using the various net related
feature flags.

tokio/tests/thread_pool.rs

@@ -274,7 +274,7 @@ fn panic_in_task() {
 
 #[test]
 fn multi_threadpool() {
-    use tokio_sync::oneshot;
+    use tokio::sync::oneshot;
 
     let pool1 = new_pool();
     let pool2 = new_pool();