enable dao-rewards-distributor to keep historic emission update logs

[bekauz]

followup to #835

currently the dao-rewards-distributor contract only tracks the latest emission schedule. this is limiting in cases where DAOs may want to update their emission schedules.

to implement that we should keep track of historic emission rate configs, along with their start dates. this way we can ensure that users can claim their earned rewards forever, and no emission rate updates will take that away from them.