[HORNETQ-1381] creates HornetQ threads in a privileged block

JIRA: https://issues.jboss.org/browse/HORNETQ-1381
Merge pull request #1754 from jmesnil/HORNETQ-1381_thread_factory_security_manager

[HORNETQ-1381] creates HornetQ threads under a security manager
HORNETQ-1383 - Implement policy to reap slow consumers automatically