fix: clear intervals on shutdown (#378)

After shutting down a circuit breaker, the status intervals are persisted and continue to run. This keeps track of each interval and shuts them down.

lib/status.js

@@ -4,6 +4,8 @@ const WINDOW = Symbol('window');
 const BUCKETS = Symbol('buckets');
 const TIMEOUT = Symbol('timeout');
 const PERCENTILES = Symbol('percentiles');
+const BUCKET_INTERVAL = Symbol('bucket-interval');
+const SNAPSHOT_INTERVAL = Symbol('snapshot-interval');
 
 const EventEmitter = require('events').EventEmitter;
 
@@ -62,20 +64,26 @@ class Status extends EventEmitter {
 
     // rotate the buckets periodically
     const bucketInterval = Math.floor(this[TIMEOUT] / this[BUCKETS]);
-    let interval = setInterval(nextBucket(this[WINDOW]), bucketInterval);
+    this[BUCKET_INTERVAL] = setInterval(nextBucket(this[WINDOW]),
+      bucketInterval);
 
     // No unref() in the browser
-    if (typeof interval.unref === 'function') interval.unref();
+    if (typeof this[BUCKET_INTERVAL].unref === 'function') {
+      this[BUCKET_INTERVAL].unref();
+    }
 
     /**
      * Emitted at each time-slice. Listeners for this
      * event will receive a cumulative snapshot of the current status window.
      * @event Status#snapshot
      * @type {Object}
      */
-    interval = setInterval(_ => this.emit('snapshot', this.stats),
+    this[SNAPSHOT_INTERVAL] = setInterval(
+      _ => this.emit('snapshot', this.stats),
       bucketInterval);
-    if (typeof interval.unref === 'function') interval.unref();
+    if (typeof this[SNAPSHOT_INTERVAL].unref === 'function') {
+      this[SNAPSHOT_INTERVAL].unref();
+    }
   }
 
   /**
@@ -154,6 +162,8 @@ class Status extends EventEmitter {
 
   shutdown () {
     this.removeAllListeners();
+    clearInterval(this[BUCKET_INTERVAL]);
+    clearInterval(this[SNAPSHOT_INTERVAL]);
   }
 }