implemented first version of simplified metadata storage API

This simplified metadata storage API connects to ns_server and
requests to ns_server's new internal _metakv API and in the end to
ns_config.

Change-Id: I80eb4c83d854c8b17e55c406bcf70b0a7436c44b
Reviewed-on: http://review.couchbase.org/44439
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>