[hdsi] Light linking improvements.

- Separate cache update and invalidation processing by queueing dirty state during notice processing.
- Compute the unioned dirty expression for evaluation over prims in the scene index. This ensures that we evaluate the path expression at most once per notice callback. The earlier version evaluated each dirty expression over prims in the scene index, which is expensive!
- Various readability improvements.

On a test shot with ~19K prims with ~20 lights using 4 unique expressions, the time spent for processing added notices reduced from 2700 ms to 390 ms.

(Internal change: 2338308)