Fix the snapshot criteria calculation (#3598)

worker/draft.go

@@ -686,7 +686,7 @@ func (n *node) checkpointAndClose(done chan struct{}) {
 					if first, err := n.Store.FirstIndex(); err == nil {
 						// Save some cycles by only calculating snapshot if the checkpoint has gone
 						// quite a bit further than the first index.
-						calculate = chk-first >= uint64(x.WorkerConfig.SnapshotAfter)
+						calculate = chk >= first+uint64(x.WorkerConfig.SnapshotAfter)
 					}
 				}
 				// We keep track of the applied index in the p directory. Even if we don't take