Make stat handling more generic

Instead of retrieving only the "vbucket-seqno" stat, make it more
generic so that other stats can be retrieved as well.

Change-Id: I283706c86cd00f45f91fb2aab8555397d4a63811
Reviewed-on: http://review.couchbase.org/34431
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>