OAK-924: Optimize namespace lookups

Avoid the need to resolve /jcr:system/rep:namespaces{/rep:nsdata}
every time a prefix mapping is looked up

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1562958 13f79535-47bb-0310-9956-ffa450edef68