Write events to storage.

[afck]

Motivation

We want to use event streams for committee changes to address the admin chain's scalability issues. Ultimately we want to replace pub-sub channels with event streams entirely. (#365)

Proposal

Write committed blocks' events to storage.

Test Plan

This doesn't use them yet, and just ports parts of #2309 to main, so we don't have to rebase them again in the future. CI should catch any regressions, and the event functionality itself will be tested in upcoming PRs, when events are actually used.

Release Plan

• Nothing to do / These changes follow the usual release cycle.

Links

• Porting parts of Store events, add a read_event method. #2309.
• Related to Replace broadcast channels by "event streams" so that producers don't need to accept subscriptions explicitly #365.
• reviewer checklist