完善事务嵌套

library/think/db/connector/Mysql.php

@@ -110,4 +110,8 @@ class Mysql extends Connection
         }
         return $result;
     }
+    
+    protected function supportSavepoint(){
+        return true;
+    }
 }

library/think/db/connector/Oracle.php

@@ -154,4 +154,8 @@ class Oracle extends Connection
     {
         return [];
     }
+
+    protected function supportSavepoint(){
+        return true;
+    }
 }

library/think/db/connector/Pgsql.php

@@ -93,4 +93,8 @@ class Pgsql extends Connection
     {
         return [];
     }
+
+    protected function supportSavepoint(){
+        return true;
+    }
 }

library/think/db/connector/Sqlite.php

@@ -92,4 +92,8 @@ class Sqlite extends Connection
     {
         return [];
     }
+
+    protected function supportSavepoint(){
+        return true;
+    }
 }