Make timestampdiff return int. (#915)

So the mysql timestampdiff documentation does not explicitly declare the
return type, but the documentation heavily implies int.

On the other hand, the MariaDB documentation explicitly specifies it as
int, so we're going to run with int.

Co-authored-by: Zephaniah E. Loss-Cutler-Hull <warp@aehallh.com>
Co-authored-by: Kyle Conroy <kyle@conroy.org>

Author: 3 people

internal/engine/dolphin/stdlib.go

@@ -5282,7 +5282,7 @@ func defaultSchema(name string) *catalog.Schema {
 					Type: &ast.TypeName{Name: "datetime"},
 				},
 			},
-			ReturnType: &ast.TypeName{Name: "datetime"},
+			ReturnType: &ast.TypeName{Name: "int"},
 		},
 		{
 			Name: "TIME_FORMAT",