[REFACTOR] Extracted routing from CDS and CDC

Extracted the routing part from the ConcurrentDocumentClient and
-Server. Both are now only responsible for transforming Jupiter-specific
Activities. (Currently Jupiter- and ChecksumActivities)

The actual routing is now performed by the ActivityHandler.

Also added some comments.

Change-Id: I80bd7b4cb88d081f20e3400bacfcd71a24c9d344
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/847
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>
Reviewed-by: Franz Zieris <zieris@inf.fu-berlin.de>