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>
(cherry picked from commit d26ba3ab0374e77847c742dd00cb3bc9301214c2)
rgw: fix crash when creating new zone on init

Moving the watch/notify init before the zone init,
as we might need to send a notification.

Reviewed-by: Sage Weil <sage@inktank.com>
Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>
(cherry picked from commit 3d55534268de7124d29bd365ea65da8d2f63e501)