MB-9462: introduced cas_local_config/2

And using it in ns_config:run_txn.

Original issue was introduced by recent switch of
ns_cluster:node_add_transaction to ns_config:run_txn.

And real bug was that ns_config:run_txn was using cas_config, which
wasn't sending changed keys to ns_config_events_local. Causing those
keys not to be replicated.

The fix is introducing cas_config version that assumes that source of
changes is local and announces changes to
ns_config_events_local. run_txn therefore fixed by using that version
of cas_config.

Change-Id: I3b89e5f0248bbbe9ece2e0f102f55bff4214ea29
Reviewed-on: http://review.couchbase.org/30115
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Artem Stemkovski <artem@couchbase.com>