reactor: rename tokio-reactor -> tokio-net (#1450)

* reactor: rename tokio-reactor -> tokio-net

This is in preparation for #1264

ui-tests/tests/ui/tokio_without_net_missing_reactor.rs

@@ -0,0 +1,3 @@
+use ui_tests::tokio::reactor;
+
+fn main() {}

ui-tests/tests/ui/tokio_without_net_missing_reactor.stderr

@@ -0,0 +1,7 @@
+error[E0432]: unresolved import `ui_tests::tokio::reactor`
+ --> $DIR/tokio_without_net_missing_reactor.rs:1:5
+  |
+1 | use ui_tests::tokio::reactor;
+  |     ^^^^^^^^^^^^^^^^^^^^^^^^ no `reactor` in `tokio`
+
+For more information about this error, try `rustc --explain E0432`.