Make Handle Send + Sync. (#35)

* Make Handle `Send + Sync`.

This is an initial implementation making `Handle: Send + Sync`. It uses
a `RwLock` to coordinate access to the underlying state storage. An
implementation without the lock is left to later.

This pass also leaves a lot of dead code that can be removed in later
commits.

* Remove reactor code related to message passing

The previous commit removed the need for using message passing to
communicate with the reactor. This commit removes all the unnecessary
code.

src/lib.rs

@@ -86,6 +86,7 @@
 #![doc(html_root_url = "https://docs.rs/tokio-core/0.1")]
 #![deny(missing_docs)]
 #![deny(warnings)]
+#![allow(unused_macros)]
 
 extern crate bytes;
 #[macro_use]
@@ -96,9 +97,6 @@ extern crate slab;
 #[macro_use]
 extern crate tokio_io;
 
-#[macro_use]
-extern crate scoped_tls;
-
 #[macro_use]
 extern crate log;