New service to manage stats trackers

This solves two problems:

1. redundant trackers making redundant requests
2. trackers being obliterated as soon as the primary metric component
   is destroyed

It introduces a new problem where visiting more and more node and
allocation pages adds to an ever-growing list of trackers that can
assume lots of memory, but it solves the problem by using a
least-recently-used cache to limit the number of trackers tracked.

ui/app/services/stats-trackers-registry.js

@@ -0,0 +1,43 @@
+import Service, { inject as service } from '@ember/service';
+import { LRUMap } from 'lru_map';
+import NodeStatsTracker from 'nomad-ui/utils/classes/node-stats-tracker';
+import AllocationStatsTracker from 'nomad-ui/utils/classes/allocation-stats-tracker';
+
+// An unbounded number of stat trackers is a great way to gobble up all the memory
+// on a machine. This max number is unscientific, but aims to balance losing
+// stat trackers a user is likely to return to with preventing gc from freeing
+// memory occupied by stat trackers a user is likely to no longer care about
+const MAX_STAT_TRACKERS = 10;
+let registry;
+
+export default Service.extend({
+  token: service(),
+
+  init() {
+    // The LRUMap limits the number of trackers tracked by making room for
+    // new entries beyond the limit by removing the least recently used entry.
+    registry = new LRUMap(MAX_STAT_TRACKERS);
+  },
+
+  getTracker(resource) {
+    if (!resource) return;
+
+    const type = resource && resource.constructor.modelName;
+    const key = `${type}:${resource.get('id')}`;
+
+    const cachedTracker = registry.get(key);
+    if (cachedTracker) return cachedTracker;
+
+    const Constructor = type === 'node' ? NodeStatsTracker : AllocationStatsTracker;
+    const resourceProp = type === 'node' ? 'node' : 'allocation';
+
+    const tracker = Constructor.create({
+      fetch: url => this.get('token').authorizedRequest(url),
+      [resourceProp]: resource,
+    });
+
+    registry.set(key, tracker);
+
+    return tracker;
+  },
+});