[FIX] #808: Inconsistencies after invitation 

This patch fixes inconsistencies after the invitation process caused by
the RemoteWriteAccessManager being informed about open editors of remote
participants to late.

This patch adds a hack to the ActivityQueuer that ensures that
missing EditorADOs are inserted into the queue before it get
flushed.

Also lets STF-Tests EditDuringInvitationTest and
EditDuringInvitationStresstest pass.

Change-Id: Ib20420ceed023c627e9e4b8c97f85d1d6c79a943
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1002
Tested-by: Jenkins CI
Reviewed-by: Patrick Schlott <pschlottk@gmail.com>