rgw/datalog: Remove use of 'detached' in `rgw_log_backing` watch

rgw/datalog: Remove use of 'detached' in `rgw_log_backing` watch

Replace `asio::detached` with `async::use_blocked` in the notify
handler. This shouldn't be too much of a problem given that our notify
handlers elsewhere use blocking `notify_ack`.

At a later time I plan to do a larger refactor, but this should be a
backportable fix for the memory error.

https://tracker.ceph.com/issues/73313
Signed-off-by: Adam C. Emerson <aemerson@redhat.com>