rgw: don't return data within the librados cb

rgw: don't return data within the librados cb

Fixes: #7030
The callback is running within a single Finisher thread, thus we
shouldn't block there. Append read data to a list and flush it within
the iterate context.

Reviewed-by: Sage Weil <sage@inktank.com>
Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit d6a4f6adfaa75c3140d07d6df7be03586cc16183)