This will be useful for the upcoming "persistent wormhole" mode. A
client might send an allocation request, crash/terminate before
receiving a response, then restart, then re-send the request. If the
server sees a request with the same request_id a previous request, it
can return the same nameplate.
We'll need code changes on both sides to support this (nothing sends or
checks request_id yet), but this lands the schema change early to reduce
future disruption.
