Implementation of the new validator disabling strategy

[tdimitrov]

Closes #1966, #1963 and #1962.

Disabling strategy specification here. (Updated 13/02/2024)

Implements:

• validator disabling for a whole era instead of just a session (been the case even before)
• no more than 1/3 of the validators in the active set are disabled
  Removes:
• DisableStrategy enum - now each validator committing an offence is disabled.
• New era is not forced if too many validators are disabled.

Before this PR not all offenders were disabled. A decision was made based on enum DisableStrategy. Some offenders were disabled for a whole era, some just for a session, some were not disabled at all.

This PR changes the disabling behaviour. Now a validator committing an offense is disabled immediately till the end of the current era.

Some implementation notes:

• OffendingValidators in pallet session keeps all offenders (this is not changed). However its type is changed from Vec<(u32, bool)> to Vec<u32>. The reason is simple - each offender is getting disabled so the bool doesn't make sense anymore.
• When a validator is disabled it is first added to OffendingValidators and then to DisabledValidators. This is done in add_offending_validator from staking pallet.
• In rotate_session the end_session also calls end_era when an era ends. In this case OffendingValidators are cleared (1).
• Then in rotate_session DisabledValidators are cleared (2)
• And finally (still in rotate_session) a call to start_session repopulates the disabled validators (3).
• The reason for this complication is that session pallet knows nothing abut eras. To overcome this on each new session the disabled list is repopulated (points 2 and 3). Staking pallet knows when a new era starts so with point 1 it ensures that the offenders list is cleared.

[tdimitrov]

And offenders list is already being cleared here:

polkadot-sdk/substrate/frame/staking/src/pallet/impls.rs

Line 512 in bbb6631

<OffendingValidators<T>>::kill();

[ordian]

And offenders list is already being cleared here:

polkadot-sdk/substrate/frame/staking/src/pallet/impls.rs

Line 512 in bbb6631

<OffendingValidators<T>>::kill();

But this is the state of offenders, not disabled state in session pallet. Although, it's currently cleared on session changes, so it's fine.

[tdimitrov]

But this is the state of offenders, not disabled state in session pallet. Although, it's currently cleared on session changes, so it's fine.

Exactly. DisabledValidators are cleared on each session. The code I linked is called in end_session just before that. I guess it should work :)

I had a concern why the validator indices are not recalculated but as far as I can see in the code, the active set is not reshuffled between sessions within an era.

The technical comments have been resolved and the PR is audited and tested