BUG-997: Evolve the SchemaRegistry concepts

This is the next step in SchemaRegistry evolution. Introduces
SchemaSourceListener and abstract classes for cache and transformer
patterns. Also completes implementation of the the abstract schema
registry and converts URLSchemaContextResolver. Finally it marks the
APIs as Beta, since they are being proven until we certify NETCONF
completely.

Change-Id: If82522c9a9a1913c818146da1247c8c5ca79b9c5
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-997: Evolve the SchemaRegistry concepts"