Domain client refactoring

[NingLin-P]

This PR brings some refactoring to the domain client, including changes of:

• Do not process the consensus chain leaves on the operator node setup
  • This was introduced so long time ago but is not needed anymore because the domain client is fork-aware, these leaves will be processed properly when their descendant block is imported
• Handling consensus block import and new slot one by one sequentially
  • In main, they are handled concurrently, if the consensus block processing is not finished (the ER is not generated) and there is a new slot hence bundle produced, the bundle will submitted along with a stale ER and cause the whole bundle to be rejected (especially when Enforce the bundles contain ER that pointing to the last domain block or extend the head receipt #1731 is implemented), so it is more rational to wait consensus block processing finish before handle new slot
  • Another way to achieve the goal is to use async Mutex to block the slot handler when there is consensus block under processing, which will bring more complexity with little performance improve
• Add testing infra support to have a way to know when all of the operator's previous tasks are finished, this will fix the domain test in Fix inherents race condition #1974

Code contributor checklist:

• I have read, understood and followed contributing guide