Datastore driver should support ID reservation

[wsh]

This is supported in the low-level client as of version 1.5, which this library already depends on. #2589 would obviate this.

[vchudnov-g]

Thanks! I categorized this issue so we can track it and address it.

[yihanzhen]

Move this issue to feature backlog and close it here. This place can continue to be used for discussion.