fix(server): Remove Socket.IO listeners

Not removing Socket.IO listeners can cause memory issues
in long running processes that spawn many Karma instance.

Fixes #2980

lib/server.js

@@ -425,6 +425,10 @@ Server.prototype._start = function (config, launcher, preprocess, fileList,
     }
 
     self.emitAsync('exit').then(function () {
+      // Remove Socket.IO listeners. `connection` callback closes over `Server`
+      // instance so it leaks Plugin state e.g. Webpack compilations.
+      socketServer.sockets.removeAllListeners()
+      socketServer.close()
       // don't wait forever on webServer.close() because
       // pending client connections prevent it from closing.
       var closeTimeout = setTimeout(removeAllListeners, webServerCloseTimeout)

test/unit/server.spec.js

@@ -65,13 +65,15 @@ describe('server', () => {
     }
 
     mockSocketServer = {
+      close: () => {},
       flashPolicyServer: {
         close: () => {}
       },
       sockets: {
         sockets: {},
         on: () => {},
-        emit: () => {}
+        emit: () => {},
+        removeAllListeners: () => {}
       }
     }