Add a query hint to limit the number of concurrent source queries

We would like a new query hint to limit the number of parallel executions of union members. This is useful in two situations:
1. When the queries are expensive, but the overall union contains a limit that is most likely to be fulfilled by the first member of the union
2. When the queries are backed by a common resource which may be overloaded by multiple queries

The addition of a hint would make this manageable by the client; perhaps something like "/+ parallel=2/"?