DDSXTY12 — Keyhash computation not well-defined for mutable types

Keyhash computation is not specified precisely enough. For instance, for mutable types it is not clear whether to use short/long parameter IDs and for PID_SENTINEL whether to set must-understand bit = 1/0? As a result, the keyhash for the same key may be different from different vendors.

Update keyhash computation for mutable types

Removed ambiguities from keyhash computation algorithm for mutable types. Specifically the following ambiguities existed.

1. Whether to use short/long parameter encapsulation
2. What's the value of must-understand bit in PID_SENTINEL.

Both ambiguities are avoided using the revised algorithm.