Clean MultiMap; pull out subclasses and provide operations that remove the need for casts in the calling code.

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1514981 13f79535-47bb-0310-9956-ffa450edef68
Track MultiMap change in ARQ.

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1514982 13f79535-47bb-0310-9956-ffa450edef68