[INTERNAL] users must now be registered in the ActivitySequencer

First step to ensure clean session shutdown and moved
the responsibility for this behavior to the upper layers.

Added a flush method to ensure that pending data is
transmitted.

TODO add shutdownOutput/Input semantics like they are
used in TCP connections.

Change-Id: Icbd7f56a583ae59d28a0dc3f73fcb42a72c9ac71
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/914
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>