Simple Staking Payouts (#5406)

* Simple Payouts

* explicit test for out of order claiming

* Add `payout_all` benchmark

* Fix merge

* add docs

* change event to controller

* Fix timestamp test warnings

* Revert "change event to controller"

This reverts commit 5d4a97832d47fe1273602d5410774d5421940c4e.

* Update Reward event doc

* Add "non-production" test

* add unlock chunk to test

* fix merge

* End payout early if no reward points

* payout_validator -> payout_stakers

* bring back payout nominator/validator, but limit their use to before migration era

* Add test for before migration

* New payout works for the era that we migrate

* Fix logic, check that migration era works

* Migrate Era tests (copypasta)

* Move comment

* Add mock back to external functions

* Fixes based on review from gui

* Update Cargo.lock

* Update Cargo.lock

* small docs update

Co-authored-by: joepetrowski <joe@parity.io>