OAK-562: Clean up duplicate name conversion in plugins/nodetype

Move classes related to node type registration to an
o.a.j.o.plugins.nodetype.write subpackage to better encapsulate the
functionality. Only the ReadWriteNodeTypeManager and InitialContent classes
need to be public.

Also replace ValidatingNodeTypeManager with a factory method in
ReadOnlyNodeTypeManager to reduce the number of separate classes.

git-svn-id: https://svn.apache.org/repos/asf/jackrabbit/oak/trunk@1434640 13f79535-47bb-0310-9956-ffa450edef68