BUG-865: do not use SchemaPath.getPath()

Gets rid of use of SchemaPath.getPath(). Also performs some drive-by
optimizations:
- use a static instance of Joiner
- use Iterables.transform() for creating path String

Change-Id: Ib5bd63a9de6c05482f0cf677f634a279f4cedcbd
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-865: do not use SchemaPath.getPath()"