BUG-994: make SchemaPath abstract

Now that the direct instantiations have been eradicated, make SchemaPath
an abstract class. Introduce RelativeSchemaPath and AbstractSchemaPath
to hold the two distinct types, eliminating the need for a boolean
field.

Change-Id: Icad3e23ac15f9238ed4c44dc77718fb43a0a9f4f
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-994: make SchemaPath abstract"