Envoys to review plan before experiment starts

[mansishr]

Adding logic for envoys to review plan before starting the experiment. Set 'review_experiment: True' under 'params' in envoy config to enable review. If any of the envoys rejects the experiment, a set_experiment_failed request is sent to the director to stop the aggregator.

[igor-davidyuk]

Could you also add a section describing this feature to the docs? And use it in one of the examples, may be just create an additional set of director/envoy configs to demonstrate

[igor-davidyuk]

This is interesting, I tested the PR and saw that Envoys do not ask for review until the Director votes (in case all of the have the review feature on) and when accepted, I got the following error: accepted.log

Another run: I declined an experiment on director and got these errors declined_director.log declined_on_collaborator.log

• processes hanged.

[igor-davidyuk]

I conducted the following experiment:

1. Sent an experiment to Director
2. Accepted it as Director
3. Declined it as Envoy
4. Sent another experiment

Result: The aggregator stopped which is good, but the new experiment never started. The first experiment's status was still running

[psfoley]

Closes https://github.com/intel/openfl/issues/386