feat: Defer cleanup for log/index compactions, add debug log

[devanbenz]

I believe that there is something happening which causes CurrentCompactionN() to always be greater than 0. Thus making Partition.Wait() hang forever.

Taking a look at some profiles where this issue occurs. I'm seeing a consistent one where we're stuck on Partition.Wait()

-----------+-------------------------------------------------------
         1   runtime.gopark
             runtime.chanrecv
             runtime.chanrecv1
             github.com/influxdata/influxdb/tsdb/index/tsi1.(*Partition).Wait
             github.com/influxdata/influxdb/tsdb/index/tsi1.(*Partition).Close
             github.com/influxdata/influxdb/tsdb/index/tsi1.(*Index).close
             github.com/influxdata/influxdb/tsdb/index/tsi1.(*Index).Close
             github.com/influxdata/influxdb/tsdb.(*Shard).closeNoLock
             github.com/influxdata/influxdb/tsdb.(*Shard).Close
             github.com/influxdata/influxdb/tsdb.(*Store).DeleteShard
             github.com/influxdata/influxdb/services/retention.(*Service).DeletionCheck.func3
             github.com/influxdata/influxdb/services/retention.(*Service).DeletionCheck
             github.com/influxdata/influxdb/services/retention.(*Service).run
             github.com/influxdata/influxdb/services/retention.(*Service).Open.func1
-----------+-------------------------------------------------------

Defer'ing compaction count cleanup inside goroutines should help with any hanging current compaction counts.

Modify currentCompactionN to be a sync atomic.

Adding a debug level log within Compaction.Wait() should aid in debugging.