VReplication: trafficswitcher's changeShardsAccess always operates on the source keyspace

[ajm188]

In changeShardsAccess, we always operate on the sourceKeyspace of the workflow here, ignoring the keyspace parameter:

func (ts *trafficSwitcher) changeShardsAccess(ctx context.Context, keyspace string, shards []*topo.ShardInfo, access accessType) error {
	if err := ts.wr.ts.UpdateDisableQueryService(ctx, ts.sourceKeyspace, shards, topodatapb.TabletType_MASTER, nil, access == disallowWrites /* disable */); err != nil {
		return err
	}
	return ts.wr.refreshMasters(ctx, shards)
}

However in allowTargetWrites, we pass targetKeyspace as the keyspace parameter here:

func (ts *trafficSwitcher) allowTargetWrites(ctx context.Context) error {
	if ts.migrationType == binlogdatapb.MigrationType_TABLES {
		return ts.allowTableTargetWrites(ctx)
	}
	return ts.changeShardsAccess(ctx, ts.targetKeyspace, ts.targetShards(), allowWrites)
}

Now, technically, this is currently fine, because we don't do any reshard workflows across keyspaces (it doesn't make sense), but that can't really be expressed in the code, and if we were to ever change that design constraint, we could get surprised by this.