Fixes a regression introduced with commit
7207bad9.

Fixes arsnova#84

* Remove unused code
* Optimize imports (group 10+ and static imports)
* Remove redundant variable assignments
* Replace explicit type parameters with <>
* Remove redundant catch blocks
* Use varargs instead of explicit array instantiation
* Weaken visibility of methods and fields
* Simplify conditions
* Do not use StringBuilder for simple concatenations
* Fix code style
* Remove invalid Javadoc

Feedback reset will be disabled for liveClicker-sessions.

Updated copyright year and adjusted header format to be consistent over
all files and with other ARSnova projects.

This should prevent concurrent ConcurrentModificationException after
application lock on undeploy.

With this patch all feedbacks will be stored in memory without any
interaction with CouchDB. Due to in memory storage, all feedbacks will
be removed if server gets restartet.

Depending on new requests, feedbacks will be deleted after some time.