CursorQueue abstraction

[sebastianst]

In #12390, the batcher got refactored to work with cursors on queues of blocks, channels and frames. Since the cursor and queue are manipulated together and consistent state between the two should be maintained, we could add a CursorQueue abstract to keep the consistency by design.

Originally posted by @sebastianst in #12390 (comment)

We may consider adding a CursorQueue abstraction that is a Queue with a cursor and then use it for blocks and frames. This way, we can better unit test the invariants between the queue and cursor.