UnackedRpcResults needed for transaction recovery may be lost during migration

When migrating a tablet, the associated UnackedRpcResults records and ParticipantList entries are migrated independently. This means that UnackedRpcResults records may be garbage collected before the TransactionManager can protect them (with UnackedRpcResults::SingleClientProtector).

This is not a problem with recovery since the recovery master can hold a UnackedRpcResults:rotector during the entire recovery while the TransactionManager puts its protectors in place.

One solution might be to hold a UnackedRpcResults:rotector during migration. However, this is more difficult since there is no simple and clear start and end of migration from the target server's perspective.