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/builder.rs

@@ -409,18 +409,18 @@ impl Builder {
         }
 
         // Create the pool
-        let inner = Arc::new(
+        let pool = Arc::new(
             Pool::new(
                 workers.into_boxed_slice(),
                 self.max_blocking,
                 self.config.clone()));
 
         // Wrap with `Sender`
-        let inner = Some(Sender {
-            inner
+        let sender = Some(Sender {
+            pool
         });
 
-        ThreadPool { inner }
+        ThreadPool { sender }
     }
 }