Add number of sstables in a compaction to compactionstats output

It would be helpful to know at a glance how many sstables are involved in any running compactions. While this information can certainly be collected now, a user has to grab it from the debug logs. I think it would be helpful for some use cases to have this information straight from nodetool compactionstats and then if the actual sstables involved in the compactions are desired, dive into the debug.log for that. I think it would also be good to have this information in the output of nodetool compactionhistory.