CBQE-1000: Partition.valid_key_set doesn't consider expired items

ValidateDataTask is broken for expired items

Change-Id: I84469c09b8b798455e5e021e82641f0a2928337f
Reviewed-on: http://review.couchbase.org/24234
Tested-by: buildbot <build@couchbase.com>
Reviewed-by: Deepkaran Salooja <deepkaran.salooja@globallogic.com>