Enhance nodetool compactionstats with existing MBean metrics

Nodetool compactionstats reports only on active compactions, if nothing is active, you see only:

$nodetool compactionstats

pending tasks: 0

but in the MBean Compaction/TotalCompactionsCompleted there are recent statistic in events/second for:

• Count
• FifteenMinueRate
• FiveMinueRate
• MeanRate
• OneMinuteRate

1) It would be useful to see in addition:

pending tasks: 0

compactions completed: 20

    1 minute rate:    0/second

   5 minute rate:    2.3/second

  15 minute rate:   4.6/second

2) Since compaction_throughput_mb_per_sec is a throttling parameter in cassandra.yaml (default 64 MBps), it would be nice to show the actual compaction throughput and be able to observe if you're close to the limit.  I.e., 

compaction throughput 13.2 MBps / 16 MBps (82.5%)

3) for completness, compactionstats should list the number of concurrent compactors configured, perhaps simply add to existing 'pending tasks' line:

4 concurrent compactors, 0 pending tasks