feat(host): impl API "/admin/pause"

[keroro520]

Add HTTP API "/admin/pause".

Clear pending tasks and cancel all running tasks and aggregation tasks when "/admin/pause" called.

• Add HTTP endpoint "/admin/pause" and "/admin/unpause"
• Add a pause flag Prover.pause_flag
• Handle pause message inside ProofActor
• Add tests covered HTTP endpoint "/admin/pause"
• Add tests covered ProofActor.handle_system_pause()
• Reject request if pause flag is true

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TODOs in the future:

• Cancelled tasks should be removed from running_tasks/aggregate_tasks
• make sure all tasks are saved to database, including pending tasks.
• feat(host): impl API "/admin/pause" #440 (comment)
• WorkingInProgress task can not be cancelled, actually.
• API for query task
• feat(host): impl API "/admin/pause" #440 (comment)

[smtmfft]

LGTM