[w3c/IndexedDB] Combining cursors / query execution engine (#298)

We have received many requests for implementing "joins". I think the way to go is to build a query executor, and ask applications to bring their own query planner and optimizer.

Conceptually, the query executor would be a new way to create cursors, which takes in an execution plan. The execution plan is an AST-like tree, where the leaves are today's cursor-creating operations, and inner nodes are operations.

A leaf has a source (object store or index) and a range (IDBKeyRange or key).

Inner node examples:
* reverse (unary)
* cross product (binary)
* hash join (binary)
* skip every N (unary)
* limit to N (unary)

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