QPID-4967 - work in progress on multi-router networks
 - Added computation of valid origins for destinations
 - Modified the forwarding algorithm to ensure that only one copy of a message is sent on
   a given inter-router link
 - Added test coverage for valid origins


git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1529242 13f79535-47bb-0310-9956-ffa450edef68