Rework Repository Entry Tracking

[adeutscher]

Primary changes:

• Repository entry states are now more synced, allowing the job repository to support a subscription for tracking the number of entries that were idempotency-blocked. This in turn allows the idempotency monitor to only loop when it is required.
• Address nomenclature drift by replacing all references of 'maintainer' in the repo with 'monitor'. The formerly-named HeartbeatMaintainer was being referred to as a monitor in places already, so this makes things more consistent.

Minor changes:

• IJobRepositoryEntry setter methods have been replaced with properties.
• For simplicity, the AppliedExecutionEndArbiter has been split up into 3 separate classes. Each is responsible for one type of background thread (executor, heartbeat monitor, idempotency monitor).

[adeutscher]

Keeping this PR in draft mode until I fix a logic hiccup in local testing, documented in #90 . It's 1AM at the moment, so this will be an issue to be resolved next session if I don't find the hiccup in the next couple minutes.