Execute PushUpdateWorker only for accounts who uses StreamingAPI just now. (#3278)

* Add redis key "subscribed:timeline:#{account.id}" to indicate active streaming API listeners exists.

* Add endpoint for notification only stream.

* Run PushUpdateWorker only for users uses Streaming API now.

* Move close hander streamTo(Http/Ws) -> stream(Http/Ws)End (Deal with #3370)

* Add stream type for stream start log message.

app/lib/feed_manager.rb

@@ -34,7 +34,7 @@ class FeedManager
       trim(timeline_type, account.id)
     end
 
-    PushUpdateWorker.perform_async(account.id, status.id)
+    PushUpdateWorker.perform_async(account.id, status.id) if push_update_required?(timeline_type, account.id)
   end
 
   def trim(type, account_id)
@@ -43,6 +43,10 @@ class FeedManager
     redis.zremrangebyscore(key(type, account_id), '-inf', "(#{last.last}")
   end
 
+  def push_update_required?(timeline_type, account_id)
+    timeline_type != :home || redis.get("subscribed:timeline:#{account_id}").present?
+  end
+
   def merge_into_timeline(from_account, into_account)
     timeline_key = key(:home, into_account.id)
     query        = from_account.statuses.limit(FeedManager::MAX_ITEMS / 4)