Reset transaction nesting level on connection loss.

When the connection is lost or is closed, subsequent transaction will no longer be nested because they started in a brand new session. Our internal representation of the nesting shold take this into account

lib/Doctrine/DBAL/Connection.php

@@ -354,6 +354,8 @@ public function connect()
         $this->_conn       = $this->_driver->connect($this->params, $user, $password, $driverOptions);
         $this->isConnected = true;
 
+        $this->transactionNestingLevel = 0;
+
         if ($this->autoCommit === false) {
             $this->beginTransaction();
         }

tests/Doctrine/Tests/DBAL/Functional/ConnectionTest.php

@@ -72,6 +72,22 @@ public function testTransactionNestingBehavior() : void
         }
     }
 
+    public function testTransactionNestingLevelIsResetOnReconnect() : void
+    {
+        $this->connection->executeQuery('CREATE TABLE test_nesting(test int not null)');
+
+        $this->connection->beginTransaction();
+        $this->connection->beginTransaction();
+        $this->connection->close(); // connection is lost
+
+        $this->connection->beginTransaction(); // should connect, reset nesting level and increase it once
+        $this->connection->executeQuery('insert into test_nesting values (33)');
+        $this->connection->rollback();
+
+        self::assertEquals(0, $this->connection->fetchColumn('select count(*) from test_nesting' ));
+
+    }
+
     public function testTransactionNestingBehaviorWithSavepoints() : void
     {
         if (! $this->connection->getDatabasePlatform()->supportsSavepoints()) {