Map predicates on min time to a portion of _id

As part of the time-series schema, min time is embedded into _id. In addition to using the control.min.time field, map min time predicates to predicates on time to the min time portion of _id as well. This should lead to guaranteed index utilization. This should be done inside $unpackBucket's doOptimizeAt() for $eq, $gt, $lt, $gte, $lte only.