Handle Missing secondary keys.

- Opaque is computed such that it gets incremented few times every second.

CBIDXT-116: CBIDXT-133:
  - Implemented missing for composite keys, provided first-item in the key
    is not empty.

Change-Id: I4d3c3c597b5bf741c35fe47f868c7d043062acfc
Reviewed-on: http://review.couchbase.org/42103
Reviewed-by: John Liang <johliang00@gmail.com>
Reviewed-by: Pratap Chakravarthy <prataprc@couchbase.com>
Tested-by: Pratap Chakravarthy <prataprc@couchbase.com>