BUG-994: introduce SchemaPath.getParent()

There are a few places which manually construct the parent of a
SchemaPath. Introduce a utility method which does precisely that in the
most efficient manner possible.

Change-Id: If144ddf3bb50f7bc25865c342ec9099045aafb93
Signed-off-by: Robert Varga <rovarga@cisco.com>
Merge "BUG-994: introduce SchemaPath.getParent()"