DISCARD ignores the DONT_LOOPBACK transient flag

When discard_all = true, DISCARD does its own loopback, and doesn't check for DONT_LOOPBACK like TP. It always sends the message back up, even if excludeItself = false.

If possible, DISCARD should just set the message destination to the local address and pass the message down. That way, TP would decide make the loopback decision, and using the TP thread pool would also make the thread name nicer in the logs. (Currently the thread name is Thread-n, which means searching for the test name in our test suite's log misses some messages.)