Simplify queueing: delete QueueWriter and QueueReader

[kosak]

The FIFO queueing between DenseStorageWriter and DenseStorageReader is needlessly complicated. We can:

1. Reduce the number of logical queues between them from 3 to 2
2. Implement queue logic directly rather than delegating to the generic QueueWriter and QueueReader.

Doing the logic directly makes the code much simpler and perhaps even more efficient.