Allocate globally unique Index Instance Id from gometa

Change-Id: I9063183b7b5cf64489492bdf1ab1e0db1f8a0f43
Reviewed-on: http://review.couchbase.org/43807
Reviewed-by: Sarath Lakshman <sarath@couchbase.com>
Reviewed-by: John Liang <johliang00@gmail.com>
Tested-by: John Liang <johliang00@gmail.com>