Audit START/END log events for CoordinatorEvent operations

[ehoner]

Operations performed by the Leader should create start/stop log pairs. These are helpful when investigating issues related to CoordinatorEvent processing. For each method corresponding to an event, the start and stop logs should begin on entry and end just before exit. The changes are itemized below according to event.

• CoordinatorEvent / handleEvent:
  • Add "END" log to precondition/early return.
• LEADER_DO_ASSIGNMENT
  • Add start and end.
  • Existing start and end capture timing only for assignment calculation. Activity before and after are not included.
• HANDLE_ADD_OR_DELETE_DATASTREAM
  • Add start and end.
• LEADER_PARTITION_MOVEMENT
  • Add start and end.