QPID-4601: HA backup broker shuts down after redeclaring exchange

There was a bug in ExchangeRegistry, it was always setting disp=created even if
the exchange was already present.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1450383 13f79535-47bb-0310-9956-ffa450edef68