State in streaming

[abramvandergeest]

Current behavior:
A variable is defined that can be written to as an output of an activity

Expected behavior:
Include a state for each operation of streaming (that can also be read by the reporting engine in issue #521)

What is the motivation / use case for changing the behavior?
Generally very useful in the DS/ML use case for being able to preserve and act on a state. Also very helpful for A/B testing.

Additional information you deem important (e.g. I need this tomorrow):