getSession throws an HTTP exception, which is not
good style. Additionally, as it calls
getSessionFromKeyword internally -- which has a caching
annotation -- this caching mechanism will not work
because it only works if the method is called
from another object. This is due to the way Spring
is using proxies to intercept the calls.

Events for all learning progress related actions
are generated, like answering a new question.
The event classes are used as a tag to evict
the cache based on the session.

There might be answers to questions without values,
those would have hit the exception. So it is better
to just ignore those questions.

Conflicts:
	src/main/java/de/thm/arsnova/controller/SessionController.java
	src/main/java/de/thm/arsnova/dao/CouchDBDao.java
	src/main/java/de/thm/arsnova/dao/IDatabaseDao.java
	src/main/java/de/thm/arsnova/services/ISessionService.java
	src/test/java/de/thm/arsnova/dao/StubDatabaseDao.java

Conflicts:
	pom.xml
	src/main/java/de/thm/arsnova/controller/SessionController.java
	src/main/resources/log4j-dev.properties

The resource by default now includes the total stats. A user parameter
has been added to request user specific data.

This change intends to restore compatibility with ARSnova Overlay.

This change also removes the "lockSession" method from CouchDB.
Instead, the more general update method is used which also
has the required caching annotations.

Note that due to the way it is currently implemented,
we have to clear the whole cache once a set of
questions published or unpublished in bulk.

Changing the Session object the way it was done previously
interferes with the new caching mechanism. The objects are
now long-lived so any direct changes to it which are not
in the spirit of its use case have to be removed. In this
case, changing the creator information invalidated many
of the isCreator checks sprinkled through the code, as
the new creator's name would have been the string
"not visible to you."