This patch intends to close all websocket connections on bean
destruction of ARSnovaSocketIOServer.

This reverts commit c22330fd.

This enables access to all UserSessionService objects from within the
application. This could be used to send events to all session users.

This patch uses an in memory storage of logged in users. Every time
users send a (online) heartbeat the last known active timestamp of all
users will be checked and inactive users will be removed from active
user list.

With this patch, websocket feedback data will be send without using
UserService arrays within ARSnovaSocketIOServer except
reportDeletedFeedback().

There is one major bug in websocket implementaton crashing the
application on websocket disconnect caused by NullPointerException.

It seems that disconnect is done after some dependencies are closed.
This might be caused by data serialisiation.

This could be used to send data via websockets without searching
sessionIDs or user tables within ARSnova.

Simply publish an event with ARSnova-sessionKey from within the
application.

Those logging entries are obsolete and only used to grow log files ;-)
Only log log entries if an error occured - not for each event.

countActiveUsers for reportActiveUserCountForSession as it was already
the case for reportSessionDataToClient.
Added check against null for session key in Socket.IO's disconnect
handler.
Logging adjustments for Socket.IO specific code.

prevent the Socket.IO server from stopping and cause a redeployment to
fail because of the still running server.

prevent duplicates if the client uses Socket.IO and still sends the
**/online ping.

LecturerQuestions and Answers to LecturerQuestions.
Renamed events concerning feedback for consistency.

should not be used by the clients.

As a side effect this should smooth the chart animation in arsnova-js.
Added some comments concerning code for legacy HTTP polling support.

UserService instead of a CouchDB request.

broadcasting it in reportSessionDataToClient method.

CouchDB and visited sessions.

Socket.IO.

requested.

interfere with the existing HTTP polling solution.

Some features still need to be ported:
- update session attribute lastOwnerActivity in CouchDB
- add session to visitedSessions in CouchDB
- check if session is locked

and the users inside of it

* rename methods to clearify that this are sockets

This provides smaller and cleaner Interfaces for each context.

All relevant information about users (e.g. session mebership) are
accessible via UserService.
The method IUserService::getCurrentUser provides information about
the actual logged in user. If no user is logged in, this method will
throw a UnauthorizedException which will end up in 401 HTTP error.