refactor: dry chain context

[mfw78]

Background

It's frustrating in that there appears to be some reasonable code duplication in the ChainContext class that is responsible for the sycnchronisation / warm up, and subsequent chain monitoring. This makes it difficult to read, reason, and regression test (maybe I just coin RRR 😆).

Details

It's desirable that the code in ChainContext be DRY'ed, with a corresponding complexity reduction. The resultant code should also be thoroughly tested.

Acceptance criteria

• Loops in ChainContext for synchronisation are simplified.
• Promote block watcher to run from the start of any sync.
• /health API endpoint returns 5XX when chain isn't in sync (remove startupProbe, and use only livelinessProbe.