MB-24645 - Improving the compaction related stats

Switching from an ever-increasing compaction stats to a
compaction cycle specific stats by reseting the stats
after every compaction
Moving back to ever increasing stat and some clean up

Change-Id: Iea58e399e9bcc4890a4f98fad10bbb082c20431a
Reviewed-on: http://review.couchbase.org/80494
Well-Formed: Build Bot <build@couchbase.com>
Reviewed-by: Steve Yen <steve.yen@gmail.com>
Tested-by: Sreekanth Sivasankaran

file.go

@@ -207,9 +207,9 @@ func newBufferedSectionWriter(w io.WriterAt, begPos, maxBytes int64,
 			if ok {
 				buf, pos = req.buf, req.pos
 				if len(buf) > 0 {
-					_, err = w.WriteAt(buf, pos)
+					nBytes, err := w.WriteAt(buf, pos)
 					if err == nil && s != nil {
-						s.reportBytesWritten(uint64(len(buf)))
+						s.reportBytesWritten(uint64(nBytes))
 					}
 				}
 			}