BUG-650: Use lazy set in YangInstanceIdentifier/SchemaPath caches

These cached values are just derived from immutable state, so we do not
need to force them to be visible by other threads -- they can calculate
them on their own and at some point one of the versions wins.

Change-Id: I3ecf272d151c693ef3adc519c33a065aa9dba94b
Signed-off-by: Robert Varga <rovarga@cisco.com>