QPID-4390: Add configuration store factory
           Use default store type if store type is not set
           Copy initial json configuration for json store instead of merging
           Remove MerginStore

git-svn-id: https://svn.apache.org/repos/asf/qpid/branches/java-broker-config-qpid-4390@1430996 13f79535-47bb-0310-9956-ffa450edef68