[INTERNAL] remove session start time from session negotiation

1. the value was not used anywhere (no longer ?) in the code
2. this value affected to many method signatures (it should not be
passed in the invitation offering but in the negotiation process)
3. XStream is a little bit too *powerful*. The value should be
transmitted as a *stupid* long value instead of a full serialized class
!


Change-Id: I1a2eceb8840a021f257b9aa82716a9b3acb9318e
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1180
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>