Bug 1985 - NPE when running groupbasedpolicy POC demo

- OFSessionUtild doesn't set session as valid anymore, session
  management moved to SessionManagerOFImpl
- session validation/invalidation is synchronized
- when registration is about to be closed SalRegistrationManager checks
  whether session from which registration is originated is valid

Change-Id: I2c523586676d3533aaf84afd2cecc38bd53181f4
Signed-off-by: Martin Bobak <mbobak@cisco.com>