Minor compaction may just happens in level 0 under heavy write pressure

[lidezhu]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

set profiles.default.dt_flush_after_write to true, and check the log to find the minor compaction process.

2. What did you expect to see? (Required)

Minor compaction can happens in all level.

3. What did you see instead (Required)

Minor compaction happens mostly in level 0.

4. What is your TiFlash version? (Required)