crimson/osd/object_context: await in-progress loading (per-obc)

crimson/osd/object_context: await in-progress loading (per-obc)

```
  // obc loading is a concurrent phase. In case this obc is being loaded,
  // make other useres of this obc to await for the loading to complete.
```

Since we now await for loading to finish (in-case in progress), we can
also assert is_loaded().

Fixes: https://tracker.ceph.com/issues/65451
Co-authored-by: Xuehan Xu <xuxuehan@qianxin.com>
Signed-off-by: Matan Breizman <mbreizma@redhat.com>