Add the ability to inspect the state of observable sequences

[glopesdev]

• Editor-integrated feature vs visualizer approach
• Timestamp all events with performance counters and reconstruct timeline afterwards
• Another idea was to use a subject that automatically collects timestamps (but this one would require merging all events)
• Debug / trace panel dedicated for this vs drawing annotations in the workflow panel
• How to keep memory of state before visualizer is open?

[glopesdev]

Maybe there are two types of "values" we want to get out of this:

• Counter-based summary of changes in state (# of subscriptions, # of notifications, # of completed sequences, etc).
• Expanded materialize node, with precisely timestamped value for every change in state (including subscription, cancellation, etc).