[EPIC] Maintain context/session state between runs

[logan-markewich]

Currently, the code that was supposed to maintain context/state between runs is all commented out.

We need to bring back this feature, by figuring out how to serialize the context. That means, we need to serialize

• the globals dict
• the queues
• etc.

In this case, I don't think it's enough to maintain just the global dict. We need it all, especially to support future use-cases where runs of a workflow can be stepwise, take days, have undo/rewind, etc.

This will require figuring out how to do this in llama-index itself first of course

[jonpspri]

I'm looking first to a simple case of managing a session-level context in the WorkflowService. I see a framework is in place but not yet implemented. The basic idea is easy, create a context for each session and persist it (in-memory initially, eventually on storage).

The simple approach is to wire that in to the existing Workflow Service or a subclass, but I wondering if the notion of a separate ContextManager class is appropriate? I'll probably hold that in store as a refactoring.