rgw: don't pass object context into async coroutines

rgw: don't pass object context into async coroutines

Fixes: http://tracker.ceph.com/issues/15625
The async read data/attrs coroutines may outlive the callers. Instead
of introducing a complicated refcounting scheme, just don't pass it in.
Anyway, it was not thread safe, and the benefit of using it is not
clear.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
(cherry picked from commit b225d1758057efc30721aad25447d54fa36e4cb2)