add subcompaction listener

[tabokie]

Signed-off-by: tabokie xy.tao@outlook.com

Add listener for subcompaction, only called when there are actually multiple subcompactions executed in parallel.
Pass in SubcompactionJobInfo containing:

  // the id of the column family where the compaction happened.
  uint32_t cf_id;
  // the name of the column family where the compaction happened.
  std::string cf_name;
  // the status indicating whether the compaction was successful or not.
  Status status;
  // the id of the thread that completed this compaction job.
  uint64_t thread_id;
  // the smallest input level of the compaction.
  int base_input_level;
  // the output level of the compaction.
  int output_level;

[Connor1996]

what's it used for?

[tabokie]

what's it used for?

@Connor1996 Check this comment: #216 (comment), used for tagging subcompaction IO type.

[yiwu-arbug]

LGTM

[Connor1996]

Add job_id so we can use it to match the CompactionJobInfo later?

[tabokie]

It seems job id is only unique within each background thread, that's why I removed it.

// the job id, which is unique in the same thread.
  int job_id;

[tabokie]

We can put more thoughts on how to match subcompaction with main compaction later. Right now this PR covers our use case.