Threadpool Info: Allow to serialize negative thread pool sizes

As a SizeValue is used for serializing the thread pool size, a negative number
resulted in throwing an exception when deserializing (using -ea an assertionerror
was thrown).

This fixes a check for changing the serialization logic, so that negative numbers are read correctly.

Closes #6325
Closes #5357
IMO we should not handle this in here... we should special case this in `ThreadPool.java` and maybe just use `null` as the value for the queue or move the `UNBOUNDED` sentinel there.