EPM and staking events improvement

[gpestana]

A few suggestions to make it easier to trace runtime events related to the election provider and staking pallets:

EPM pallet

• SolutionStored { compute, prev_ejected, origin, Option<T::AccountId> } (modified).
• PhaseTransitioned { from: Phase, to: Phase, round: u32}: All the phase transition events (Event::SignedPhaseStarted, Event::UnsighedPhaseStarted) are replaced by a state transition event that keep the previous and next phase. This will also allow us to easily start emitting when the emergency and phase off start and all new future phases that we may need.

Staking pallet:

• ForceEra { mode: Forcing }: signals that a new Forcing mode was set.

In addition to refactoring and adding new events, this PR also adds helper functions to transition EPM phases and set a new force era mode.

[Ank4n]

• SolutionStored { compute, prev_ejected, account_id, score }: should also have the AccountId of the miner that stored the solution and its score.

• FYI: this event is also emitted for unsigned solutions. We can keep it as optional which is then will also be useful to indicate whether it was a signed or an unsigned solution.

👍

• EmergencyPhaseStarted: indicates when the emergency phase has started.
  👍 . I think we should have an event for the start of each phase including even Off Phase?

Yes, I think that would be helpful.

• FallbackSolution: indicates that a solution has been provided by T::Fallback
• GovernanceSolution: indicates that a solution has been provided by T::GovernanceFallback
  Might be better to indicate this in the existing SolutionSotred event?

Right, the solution type is already stored as compute as param of the Event::Finalized.