update-replace-synonym

Summary:

- `UPDATE` statements now support `REPLACE` as the verb.
- This is in support of nuanced routing in `stackql`.

Author: general-kroll-4-life

go/vt/sqlparser/ast.go

@@ -144,6 +144,7 @@ type (
 	// Update represents an UPDATE statement.
 	// If you add fields here, consider adding them to calls to validateUnshardedRoute.
 	Update struct {
+		Action     string
 		Comments   Comments
 		Ignore     string
 		TableExprs TableExprs

go/vt/sqlparser/constants.go

@@ -38,6 +38,7 @@ const (
 	// DDL strings.
 	InsertStr  = "insert"
 	ReplaceStr = "replace"
+	UpdateStr  = "update"
 
 	// Set.Scope or Show.Scope
 	SessionStr        = "session"