[FIX] activity queuing is now only enabled when the session blocked

This patch ensures that all users are actually stopped (no resource
activities should be generated) when the remote side starts
activity queuing during the project negotiation.

As a result it should now be impossible to miss resource activities
which may lead to an inconsistent session state.

Change-Id: I535ea0b3e19c719173d4f4d43af3d12cb6ca074d
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/928
Tested-by: Jenkins CI
Reviewed-by: Patrick Schlott <pschlottk@gmail.com>
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>