MB-100: implemented ns_config:run_txn

Which implements optimistic transactions on top of cas_config. And
thus avoids running arbitrary and potentially dangerous code inside
ns_server.

Change-Id: I59bf6a192baad4a6c6f370a381931111df81ec7c
Reviewed-on: http://review.couchbase.org/29865
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>
MB-100: killed online config upgrade for xdcr settings

Which was never really working

Change-Id: I80baa22cb73d3a0c3d40bcd8ece6b08a20fcfb71
Reviewed-on: http://review.couchbase.org/29866
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>