The usage of var_export can cause circular reference and memory leak on internal logic

Problem/Motivation

The usage of var_export does not allow circular references. If any class for internal logics is added to $message the rerouting will result in a memory leak.

Steps to reproduce

Add a class, for instance TranslatableMarkup to an array key of $message (e.g. $message['body'][]. The key $message['subject'] however has a workaround.

Proposed resolution

Use another serialization method to log the $message array to Watchdog.