Add SQL commands to disable constraints to MySQL and SQLite.

Postgres and SQLserver don't seem quite so simple.

src/Database/Dialect/MysqlDialectTrait.php

@@ -66,12 +66,14 @@ trait MysqlDialectTrait {
  * {@inheritDoc}
  */
 	public function disableForeignKeySQL() {
+		return 'SET foreign_key_checks = 0';
 	}
 
 /**
  * {@inheritDoc}
  */
 	public function enableForeignKeySQL() {
+		return 'SET foreign_key_checks = 1';
 	}
 
 }

src/Database/Dialect/SqliteDialectTrait.php

@@ -163,12 +163,14 @@ trait SqliteDialectTrait {
  * {@inheritDoc}
  */
 	public function disableForeignKeySQL() {
+		return 'PRAGMA foreign_keys = OFF';
 	}
 
 /**
  * {@inheritDoc}
  */
 	public function enableForeignKeySQL() {
+		return 'PRAGMA foreign_keys = ON';
 	}
 
 }