Switch to Apache HttpComponents HttpClient to poll feeds over HTTP.

This introduces an additional dependency over the library function for opening a stream from a URL, but gets us Keep-Alive support, compression, and caching with ETag and timestamps, all of which make polling more efficient.
Fix bug where we attempted to close a WebSocket which had already been closed.
Remove unused method GtfsRealtimeFeed.getAllFeedEntities().
Fix typo in CombinedEntityListener.