crimson/.../client_request_common: simplify recover_missings

crimson/.../client_request_common: simplify recover_missings

- Modify ClientRequest::snaps_need_to_recover() to return all
  relevant snaps including the operation target.
- Update ClientRequest::snaps_need_to_recover() and
  CommonClientRequest::recover_missings to use set::set
  for snaps.
- Remove special handling for soid from
  CommonClientRequest::recover_missings
- Simplify CommonClientRequest::recover_missings.

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