fix(lastn) debounce updating last N

When joining a large meeting every participant join would trigger an attempt of
an update. Try to minimize the amount of calls.

react/features/base/lastn/middleware.js

 // @flow
 
+import debounce from 'lodash/debounce';
+
 import { SET_FILMSTRIP_ENABLED } from '../../filmstrip/actionTypes';
 import { SELECT_LARGE_VIDEO_PARTICIPANT } from '../../large-video/actionTypes';
 import { APP_STATE_CHANGED } from '../../mobile/background/actionTypes';
 import { limitLastN } from './functions';
 import logger from './logger';
 
-declare var APP: Object;
-
-MiddlewareRegistry.register(store => next => action => {
-    const result = next(action);
-
-    switch (action.type) {
-    case APP_STATE_CHANGED:
-    case CONFERENCE_JOINED:
-    case PARTICIPANT_JOINED:
-    case PARTICIPANT_KICKED:
-    case PARTICIPANT_LEFT:
-    case SCREEN_SHARE_REMOTE_PARTICIPANTS_UPDATED:
-    case SELECT_LARGE_VIDEO_PARTICIPANT:
-    case SET_AUDIO_ONLY:
-    case SET_FILMSTRIP_ENABLED:
-    case SET_TILE_VIEW:
-        _updateLastN(store);
-        break;
-    }
-
-    return result;
-});
-
 /**
  * Updates the last N value in the conference based on the current state of the redux store.
  *
  * @private
  * @returns {void}
  */
-function _updateLastN({ dispatch, getState }) {
+const _updateLastN = debounce(({ dispatch, getState }) => {
     const state = getState();
     const { conference } = state['features/base/conference'];
-    const { enabled: audioOnly } = state['features/base/audio-only'];
-    const { appState } = state['features/background'] || {};
-    const { enabled: filmStripEnabled } = state['features/filmstrip'];
-    const config = state['features/base/config'];
-    const { lastNLimits, lastN } = state['features/base/lastn'];
-    const participantCount = getParticipantCount(state);
 
     if (!conference) {
         logger.debug('There is no active conference, not updating last N');
         return;
     }
 
+    const { enabled: audioOnly } = state['features/base/audio-only'];
+    const { appState } = state['features/background'] || {};
+    const { enabled: filmStripEnabled } = state['features/filmstrip'];
+    const config = state['features/base/config'];
+    const { lastNLimits, lastN } = state['features/base/lastn'];
+    const participantCount = getParticipantCount(state);
+
     // Select the lastN value based on the following preference order.
     // 1. The last-n value in redux.
     // 2. The last-n value from 'startLastN' if it is specified in config.js
 
     logger.info(`Setting last N to: ${lastNSelected}`);
     dispatch(setLastN(lastNSelected));
-}
+}, 1000); /* Don't send this more often than once a second. */
+
+
+MiddlewareRegistry.register(store => next => action => {
+    const result = next(action);
+
+    switch (action.type) {
+    case APP_STATE_CHANGED:
+    case CONFERENCE_JOINED:
+    case PARTICIPANT_JOINED:
+    case PARTICIPANT_KICKED:
+    case PARTICIPANT_LEFT:
+    case SCREEN_SHARE_REMOTE_PARTICIPANTS_UPDATED:
+    case SELECT_LARGE_VIDEO_PARTICIPANT:
+    case SET_AUDIO_ONLY:
+    case SET_FILMSTRIP_ENABLED:
+    case SET_TILE_VIEW:
+        _updateLastN(store);
+        break;
+    }
+
+    return result;
+});