Create nested class VCMTPReceiver::BatchedNotifier

Change field VCMTPReceiver::notifier from reference to object. Undeclare
VCMTPReceiver inheriting from ReceivingApplicationNotifier.  All done to
defensively copy the notifier (so that the client may delete) and
avoid double destruction of the VCMTPReceiver instance due to inheriting from
ReceivingApplicationNotifier.

The notification methods must still be implemented. They are now
    VCMTPReceiver::BatchedNotifier::notify_of_bof
    VCMTPReceiver::BatchedNotifier::notify_of_eof
    VCMTPReceiver::BatchedNotifier::notify_of_missed_file