Provide an easy way to parse/retrieve facets

Current facet results are returned within the rep:facet($propertyname) property of each resulting node. The resulting String [1] is however a bit annoying to parse as it separates label / value by comma so that if label contains a similar pattern parsing may even be buggy.
An easier format for facets should be used, eventually together with an utility class that returns proper objects that client code can consume.

[1] : https://github.com/apache/jackrabbit-oak/blob/trunk/oak-lucene/src/test/java/org/apache/jackrabbit/oak/jcr/query/FacetTest.java#L99