IF: Modify set_proposed_producers() to make sure propser_schedule.version numbers not being skipped

[linh2931]

In reviewing #2282, @heifner commented:

I believe this change means it is now possible to skip propser_schedule.version numbers. We should modify the set_proposed_producers logic so we don't skip version numbers. Need a test to cover that case so we don't break it again.

We need to make sure that if a proposer policy is scheduled for the same proposer_policy.active_time that it replaces the current one there but doesn't increment the schedule version. I think we will need to change controller::set_proposed_producers to check if there is an existing one that would have the same active_time and replace it and use its version. We should return the correct version to the contract.

Please see #2282