`Log4jLog` needs to re-resolve `ExtendedLogger` on deserialization (for compatibility with Log4J 2.21)

[jhoeller]

Log4J 2.21 holds a ThreadLocal instance in a non-transient field which breaks serialization behind our Commons Logging adapter. Since we re-resolve the delegate on deserialization for SLF4J already, we should do the same for Log4J.