[hooks] fix timestamp confusion in TransactionsCleanupStartupHook transactions.tx_time is inserted as datetime.utcnow(), in server/sources/native.py:NativeSQLSource.start_undoable_transaction. Don't compare it to datetime.now().

from yams.buildobjs import RelationDefinition

class ARelation(RelationDefinition):
    subject = 'CWUser'
    object = 'CWGroup'