executor: fix point get in pessimistic txn (pingcap#5)

In pessimistic txn, statement retry should use 'for update ts' to read
but point get use the snapshot with start ts

Author: tiancaiamao

executor/adapter.go

@@ -438,6 +438,9 @@ func (a *ExecStmt) handleNoDelayExecutor(ctx context.Context, sctx sessionctx.Co
 				return nil, errors.Trace(err)
 			}
 
+			// Rollback the statement change before retry it.
+			sctx.StmtRollback()
+
 			if err = e.Open(ctx); err != nil {
 				return nil, errors.Trace(err)
 			}

executor/point_get.go

@@ -137,9 +137,17 @@ func (e *PointGetExecutor) get(key kv.Key) (val []byte, err error) {
 	if err != nil {
 		return nil, errors.Trace(err)
 	}
+
 	if txn != nil && txn.Valid() && !txn.IsReadOnly() {
-		return txn.Get(key)
+		val, err = txn.GetMemBuffer().Get(key)
+		if err == nil {
+			return val, err
+		}
+		if !kv.IsErrNotFound(err) {
+			return nil, err
+		}
 	}
+	// In pessimistic txn, this snapshot may be using the 'for update ts' instead of start ts.
 	return e.snapshot.Get(key)
 }