use sync.Once for cache initialization
use sync.Once for cache initialization

Change-Id: Icfa22a7bfbd6ab494dee894b0700a293f1a50d6e
Reviewed-on: http://review.couchbase.org/58917
Tested-by: Artem Stemkovski <15a0730a27c69f62ab86f2b6c639fca202166c02@couchbase.com>
Reviewed-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>

couchbase/cbauth,couchbase/cbauth