Moved some cache related classes to org.jivesoftware.util.cache. Made Cache an interface and moved its implementation down into DefaultCache. Initial pass at porting CacheFactory from clearspace.

git-svn-id: http://svn.igniterealtime.org/svn/repos/openfire/trunk@8139 b35dd754-fafc-0310-a699-88a17e54d16e