Expose Saved Object API to Server (elastic#12182)

* expose SavedObjectsClient to server

* update prereqs function to use getSavedObjectsClient request decoration

* fix indentation

src/server/saved_objects/saved_objects_mixin.js

@@ -13,13 +13,7 @@ export function savedObjectsMixin(kbnServer, server) {
     getSavedObjectsClient: {
       assign: 'savedObjectsClient',
       method(req, reply) {
-        const adminCluster = req.server.plugins.elasticsearch.getCluster('admin');
-        const callAdminCluster = (...args) => adminCluster.callWithRequest(req, ...args);
-
-        reply(new SavedObjectsClient(
-          server.config().get('kibana.index'),
-          callAdminCluster
-        ));
+        reply(req.getSavedObjectsClient());
       }
     },
   };
@@ -29,4 +23,22 @@ export function savedObjectsMixin(kbnServer, server) {
   server.route(createFindRoute(prereqs));
   server.route(createReadRoute(prereqs));
   server.route(createUpdateRoute(prereqs));
+
+  const savedObjectsClientCache = new WeakMap();
+  server.decorate('request', 'getSavedObjectsClient', function () {
+    const request = this;
+
+    if (savedObjectsClientCache.has(request)) {
+      return savedObjectsClientCache.get(request);
+    }
+
+    const { callWithRequest } = server.plugins.elasticsearch.getCluster('admin');
+    const callAdminCluster = (...args) => callWithRequest(request, ...args);
+    const savedObjectsClient = new SavedObjectsClient(
+      server.config().get('kibana.index'),
+      callAdminCluster
+    );
+    savedObjectsClientCache.set(request, savedObjectsClient);
+    return savedObjectsClient;
+  });
 }