Brand new implementation of HostController.start.

The new implementation differs from the old on the following ways:

- Has a promise-based API.
- Based on promises thoughout.
- Reads from top to bottom.
- Lots of comments.
- Data flow is (once again) completely explicit.
- Runs async operations in parallel when possible.  Order of operations
  was overspecified in the previous version.
- Miraculously still passes unit tests.

BUG=471928

Review URL: https://codereview.chromium.org/1081603002

Cr-Commit-Position: refs/heads/master@{#324972}

remoting/webapp/crd/js/crd_main.js

@@ -133,7 +133,8 @@ remoting.updateLocalHostState = function() {
   };
 
   remoting.hostController.hasFeature(
-      remoting.HostController.Feature.PAIRING_REGISTRY, onHasFeatureResponse);
+      remoting.HostController.Feature.PAIRING_REGISTRY).
+      then(onHasFeatureResponse);
   remoting.hostController.getLocalHostState(onHostState);
 };