avoid taking the PBWM lock in SessionsCollectionRS::_isStandaloneOrPrimary()

SessionsCollectionRS::_isStandaloneOrPrimary() is used by the PeriodicJob for the LogicalSessionCache on both primaries and secondaries. Because this check relies implicitly on the PBWM lock, this process may be blocked on slow oplog application. It may be possible to modify this function to not require the PBWM lock.