librbd: fix a bunch of issues with restarting RefreshRequest

librbd: fix a bunch of issues with restarting RefreshRequest

Make RefreshRequest properly restartable, at least up until and including
V2_REFRESH_PARENT step:

- clear m_migration_spec when skipping GET_MIGRATION_HEADER
- don't rely on potentially stale m_incomplete_update on retry
- reset m_legacy_parent when retrying more than just V2_GET_PARENT
- don't rely on potentially stale m_parent_md.overlap and
  m_head_parent_overlap on retry
- clear m_metadata before fetching image metadata (but not before
  fetching pool metadata)
- clear m_op_features when skipping V2_GET_OP_FEATURES
- clear m_group_spec on EOPNOTSUPP error in V2_GET_GROUP
- reset m_legacy_snapshot when retrying more than just V2_GET_SNAPSHOTS
- don't rely on potentially stale m_snap_parents on retry

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>