Change AQ semantics for attributes with raw value

For attribute types with `needs_raw_value = True`, the AQ query
semantics are now:

* For string operations (e.g., AC, STARTSWITH, ENDSWITH, CONTAINS), AQ
  queries use the raw value.

* For all other operations (e.g., EQ, GT, ...) AQ queries use
  the cooked value.

* The new AQ operations EQS and NES support equal and non-equal
  comparison of raw values.