rgw: change cache / watch-notify init sequence

Fixes: #6046
We were initializing the watch-notify (through the cache
init) before reading the zone info which was much too
early, as we didn't have the control pool name yet. Now
simplifying init/cleanup a bit, cache doesn't call watch/notify
init and cleanup directly, but rather states its need
through a virtual callback.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>