Refactor PolicyMonitor

[wolf4ood]

Feature Request

Currently the PolicyMonitor is implemented as a normal state machine like TransferProcess and ContractNegotiation one.

But as stated here the PolicyMonitor implemented as SM could be overkill and not optimal in terms of performance and database load.

Since the only non final state of PolicyMonitor is STARTED we could implement it more as a watchdog than a state machine
and implement an optimized strategy for watching the entry in STARTED state.

[github-actions]

This issue is stale because it has been open for 14 days with no activity.

[github-actions]

This issue is stale because it has been open for 28 days with no activity.