Misc fixes

1. Connecting to dispyscheduler from the client (SharedJobCluster) with a keyfile/certfile didn't work.
2. Why did dispynode block PING messages from the scheduler IP? I've removed the check.
3. When the job submit rate was high enough, the _job object was reused, causing a clash of uid's. I've changed this particular case to using uuid.uuid4().hex, but you might want to go over the other cases and do something similar. Also, using os.urandom as a "hash" seems wrong. Better to calculate it with hash() if a hash is actually needed, or generate a UUID.
4. When stdin was /dev/null, e.g. when started as a systemd service unit, the sys.stdin.readline() in the main loop caused a 100 % CPU busy loop.