remove files from filesCompacting when clear compaction queues

When clear compaction queues, we just clear the workQueue of ThreadPoolExecutor, but files in compaction request are still in filesCompacting list. maybe we should clear it also.

For system compaction, selectNow will be false, which means files will be added to the compacting list when the compaction task is actually executed. So it is safe to just clear the workQueue of compaction thread pool executor.

However, for user-triggered compaction, selectNow will be true, which means files are already added to the compacting list when we put the compaction request to the queue. When CompactionPolicy selects candidate files for minor compaction, files in compacting list will be excluded. If we just clear the workQueue of compaction thread pool executor, files of these compaction requests are still in compacting list. These files will not be selected by CompactionPolicy and compacted any more unless we force a major compaction. So I think maybe we should remove those files from compacting list when we clear the compaction queue.