Create a new event VALIDATOR_CHANGED in lisk-chain

[nazarhussain]

Description

• Add EVENT_VALIDATORS_UPDATED in lisk-chain when setValidators is called. the payload should be { validators: Validator[] } which is set.
• Emit channel event chain:validators:update by listening to the above event. The payload must be in JSON format. (ie: address Buffer must be converted to Base64 string)

Motivation

In lisk-dpos we had an event emitter implemeation for EVENT_ROUND_CHANGED. This does not seems to be relevant any more for the new dpos module. We rather have a different event EVENT_VALIDATOR_CHANGED.

https://github.com/LiskHQ/lisk-sdk/blob/0bd21816e1e1e42a7abaf3619eb37a3f8a915e69/elements/lisk-dpos/src/delegates_info.ts#L144-L147

Round information is no more relevant to on-chain architecture so any of such event should be removed or replaced.

Acceptance Criteria

• Add unit test to see if the event is emitted on lisk-chain