doc/dev/osd_internals:manifest.rst: clarification for manifest design and future...

doc/dev/osd_internals:manifest.rst: clarification for manifest design and future steps

I've added a few significant changes:
* TIER_PROMOTE purely ensures that data is resident in the base pool
* Add EVICT_CHUNK to permit a tiering agent to selectively cold space
  from HEAD/snapshots independently of SET_CHUNK.
* Avoid using DIRTY entirely for dedup targets.
* Instead of modifying clone_range when updating clone manifests, simply
  update ReplicatedBackend::calc_*_subsets to also consider the clone
  obc manifest.

I've also added sections on how tiering agents are rbd/rgw are meant
to interact as well as on testing.

Signed-off-by: Samuel Just <sjust@redhat.com>