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.

tokio-threadpool/src/notifier.rs

@@ -13,7 +13,7 @@ use futures::executor::Notify;
 /// to poll the future again.
 #[derive(Debug)]
 pub(crate) struct Notifier {
-    pub inner: Arc<Pool>,
+    pub pool: Arc<Pool>,
 }
 
 /// A guard that ensures that the inner value gets forgotten.
@@ -38,7 +38,7 @@ impl Notify for Notifier {
                 // Bump the ref count
                 let task = task.clone();
 
-                let _ = self.inner.submit(task, &self.inner);
+                let _ = self.pool.submit(task, &self.pool);
             }
         }
     }