threadpool: rename inner to something more descriptive (#768)

`inner` is a fitting name for variables of type named `Inner`, but in other cases I find them confusing - sometimes `inner` refers to a `Pool`, sometimes to a `Sender`. I renamed a bunch of variables named `inner` to be more descriptive.

This PR is the first step in an effort of splitting https://github.com/tokio-rs/tokio/pull/722#issuecomment-439552671 into multiple PRs.

src/runtime/mod.rs

@@ -153,7 +153,7 @@ pub struct Runtime {
 
 #[derive(Debug)]
 struct Inner {
-     /// A handle to the reactor in the background thread.
+    /// A handle to the reactor in the background thread.
     reactor_handle: Handle,
 
     // TODO: This should go away in 0.2