Decrease Compactor heartbeat interval to 8 seconds

[Lujie1996]

Currently checkpointer heartbeat is checked every 10 seconds, but it's updated by checkpointer every 15 seconds, resulting in noisy logs (i.e. HeartBeat not moving forward for table).

Checklist (Definition of Done):

• [x] There are no TODOs left in the code
• [x] Coding conventions (e.g. for logging, unit tests) have been followed
• [x] Change is covered by automated tests
• [x] Public API has Javadoc