[pixels-common] support per-query I/O scheduling.

[bianhq]

Currently, I/O scheduling strategies are global, i.e., they can not do dedicated scheduling for specific queries.

Per-query I/O scheduling is useful. For example, giving priority to different queries, or diable request merging for the first few request batches of each query.

[bianhq]

We implemented per-query scheduling, but it does not provide any performance gain.
However, the query id has been passed into the scheduler. So we can add a better per-query scheduling policy in the future.