storage: avoid one extra round of wait

It could exit early if it knows that there is no more keys.

storage/kvstore_compaction.go

@@ -31,7 +31,7 @@ func (s *store) scheduleCompaction(compactMainRev int64, keep map[revision]struc
 			}
 		}
 
-		if len(keys) == 0 {
+		if len(keys) < int(batchsize) {
 			rbytes := make([]byte, 8+1+8)
 			revToBytes(revision{main: compactMainRev}, rbytes)
 			tx.UnsafePut(metaBucketName, finishedCompactKeyName, rbytes)