Remove migration id from traffic purchases

At this point, having to pass in the right migration id for traffic purchases just seems confusing as noone even knows what a migration id is anymore.

The current code works by passing in the migration id through BuyMemberTraffic (or the token std equivalent). This then gets copied to the MemberTraffic contract.

The filtering by migration id happens in the SvDsoStore ingestion.

We can't just remove the migration id filter as then we'll pick up the old MemberTraffic contracts that have never been archived.

The best option I can come up with is the following:

1. Define a special migration id (e.g. 100)
2. Change the ingestion to ingest 100 + the configured migration id (so the sv config retains the migration id but that's fine)
3. Change the Daml code to only produce MemberTraffic contracts with the special migration id regardless of what the user specifies.