[D7] Add ability to register rootEndTransaction callbacks during transactions

In DatabaseConnection, the patch adds this code.

+  /**
+   * Do the actual commit, invoke pre-commit callbacks.
+   *
+   * @internal
+   */
+  protected function doCommit() {
+    $success = $this->connection->commit();
+    $this->executeRootTransactionEndCallbacks($success);
+
+    if (!$success) {
+      throw new DatabaseTransactionCommitFailedException();
+    }
+  }

In DatabaseConnection_mysql, the code is changed as follows.

   protected function doCommit() {
     if ($this->connection->inTransaction()) {
-      return $this->connection->commit();
+      $success = parent::doCommit();
     }
     else {
       // In PHP 8.0 a PDOException is thrown when a commit is attempted with no
       // transaction active. In previous PHP versions this failed silently.
-      return TRUE;
+      $success = TRUE;
+      // Allow callbacks to perform their own cleanup.
+      $this->executeRootTransactionEndCallbacks($success);
     }
+    return $success;
   }

$success = parent::doCommit(); would just set $success to NULL, since DatabaseConnection::doCommit() does not return any value.