Replace random class_tracker_id with sequential integer.

[aebrahim]

We keep the same 32 digit string hexadecimal format.

Fixes #510.
Relevant to #453 and apache/beam#21298

[claudevdm]

Hi @aebrahim , this approach leads to errors in Apache Beam.

For example, in Apache beam

1. cloudpickle is used to serialize dynamic types during pipeline submission time (could be on a user's workstation)
2. distributed workers also use cloudpickle to serialize dynamic types

During step (1) some user defined type will claim the tracking id 1
During step (2) the worker imports cloudpickle library and serializes some dynamic type, but it has no idea that tracking id 1 was already claimed in a separate python process. Now there are two types encoded with the tracking id 1.

[aebrahim]

Got it, so the approach taken needs to be some kind of deterministic hash?

[claudevdm]

Yes. Perhaps there are ways to strike a balance here depending on the use case

• The user of cloudpickle can pass their own "generate id" function to be used when pickling a dynamic type, while the default stays uuid (which is the proven way to guarantee isinstance semantics)
• There could be an option to forego isinstance semantics completely (dont use tracking id's at all and always create a new type) when determinism is preferred vs isinstance semantics