app: simplify end-epoch handling using the sct component

[erwanor]

Is your feature request related to a problem? Please describe. With #3703, we fleshed out a SCT component that supports managing epochs and block height/timestamps. This unlocks follow-up work on the same track as #3117, namely, removing all ad-hoc handling of epochs in the app code and encapsulate "clock ticks" into the SCT component.

This will require a somewhat gnarly rework of how we handle end_block and finalize compact blocks. However, this shouldn't be interface breaking and can be done in immediate follow-up work or in an upgrade.