Several more fixes for multi-level timing coherence.

Several more fixes for multi-level timing coherence.
- Add "deferred snoop" flag to Packet so upper-level caches
  can distinguish whether lower-level cache request was
  in-service or not at the time of the original snoop.
- Revamp response handling to properly handle deferred snoops
  on non-cache-fill requests (i.e. upgrades).
- Make sure forwarded writebacks are kept in write buffer at
  lower-level caches so they get snooped properly.

--HG--
extra : convert_revision : 17f8a3772a1ae31a16991a53f8225ddf54d31fc9