[INTERNAL] changed activity receive logic

The activities are already in order. There is no need to buffer them. A
plain check if the sequence numbers are matching is enough.

Change-Id: If2a4f0c768b40fe94977b3eb8ceae88648b9d683
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1767
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>