Fix for #1868 - make State subclasses real classes

[noffke]

make State subclasses real classes (not objects, i.e. singletons) so that each request and response can have their own state instance

Description

State subclasses are objects which make them singletons. Hence, all requests and responses share the same state instance. I think the subclasses should be classes, so that requests and responses have their own State instances.

Motivation and Context

Please see #1868

Types of changes

• [x ] Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• [ ?] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• [ x] I have added tests to cover my changes.

[kasmarian]

Thank you for the fix, @noffke !