sc: support dynamic configs (for backfill)

stub

(might be particularly useful for monitoring backflow, so we can dynamically adjust a backflowMessagesPerHour timer and/or the blockUnsealedMessages toggle based on current conditions)

• we could imagine a DynamicConfigs(app) class backed by a redis store defining default values for configs that could be tweaked on the fly from a redis console
• it would have fields for toggles, knobs, and switches
• the above could live here, as could things that are currently static timer configs