Fix: Canvas socket auth (#24094)

## Summary

Closes https://github.com/elastic/kibana/issues/23303 ~(@cqliu1 can you confirm this too?)~ confirmed

Fixes the way we capture the request info when configuring the socket and providing it to plugins via `callWithRequest`. Instead of exposing a route that returns the info, simply use the request object that comes back from `server.inject`.

Also adds a check in the `elasticsearchClient` handler exposed to plugins to ensure the session is still valid because using `callWithRequest`.

![screenshot 2018-10-16 10 37 56](https://user-images.githubusercontent.com/404731/47036828-32768c00-d132-11e8-81a0-122b5e83c7ef.png)
*Note:* the actual error message is a bit different, but this is how the failure is exposed to the user

x-pack/plugins/canvas/server/lib/get_request.js

@@ -0,0 +1,17 @@
+/*
+ * Copyright Elasticsearch B.V. and/or licensed to Elasticsearch B.V. under one
+ * or more contributor license agreements. Licensed under the Elastic License;
+ * you may not use this file except in compliance with the Elastic License.
+ */
+
+export function getRequest(server, { headers }) {
+  const basePath = server.config().get('server.basePath') || '/';
+
+  return server
+    .inject({
+      method: 'GET',
+      url: basePath,
+      headers,
+    })
+    .then(res => res.request);
+}