CBIDXT-83 Move all constants to constant.go

Change-Id: Ibbe3422064d11f327e7a61d9839efd669a59cc77
Reviewed-on: http://review.couchbase.org/38867
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Reviewed-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>
Tested-by: Deepkaran Salooja <deepkaran.couchbase@gmail.com>