Revert "MB-9096: killed ns_online_config_upgrader"

This reverts commit 6c13b6f64379b21e83429c149d4399b7c486fcea.

It turns out 2.5 needs this back. In order to simplify merge it looks
worth reviving that guy.

In addition to simply reverting it, I've removed "on-join" config
upgrade and made it handle "upgrade" to 3.0.

Change-Id: Id2320ede37e6145008a35a22867421a29e83e46c
Reviewed-on: http://review.couchbase.org/29933
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>