Lazy payouts (#4474)

* TODOs

* Remove superfluous:

* partial implementation

* full implementation

* fix preferences

* update comments

* upgrade test WIP

* fix more tests

* fix cutoff

* fix saturation

* comment

* upgrade mock

* upgrade test

* WIP migration

* WIP migration

* remove slot stake stuff

* fix merge

* migration of ledger

* remove equalize from test

* add test

* fix

* update doc

* fix compilation

* improve test readibility

* improve doc

* fix most todo

* fix migration and test

* remove println

* WIP

* add test and spec

* weight

* update doc

* safer end_era

* fix exposure of conversion

* Revert "safer end_era"

This reverts commit 72ff737d27be67d87308514b13e2574bc5f09fce.

* fix useless put

* exposure clipped

* doc

* fix payout with clipped

* fix node runtime

* add doc

* pluggable and generalized staking module

* remove print

* update doc

* refactor

* improve documentation and implementation

* fix test

* Fix test

* fix test

* fix test

* fix remove lowest stake from exposure, not biggest.

* nomination index arguments in nominator_payout

* add test

* try to fix offence

* apply slashed and bond eras until active era

* doc

* update spec version

* add test upgrade from previous test environment

* Apply suggestions from code review

Co-Authored-By: Shawn Tabrizi <shawntabrizi@gmail.com>

* nominators upgrade has been cleaned

* dynamic history depth implementation

* make current_era - history_depth included

* Change equality check to start era to less than or equal

* Use era specific validator prefs

* Add print statement and comment about start era if <

* fix next_reward overflow

* make more check for bad era claim for zero cost

* small refactor

* code refactor + fix use of deprecated storage

* fix wasm build

* add comment

* Fix tests

* remove outdated comment

* Apply suggestions from code review

Co-Authored-By: Shawn Tabrizi <shawntabrizi@gmail.com>

* gather active era information into one storage

Co-authored-by: thiolliere <gui.thiolliere@gmail.com>
Co-authored-by: Shawn Tabrizi <shawntabrizi@gmail.com>

substrate/bin/node/cli/src/chain_spec.rs

@@ -247,7 +247,6 @@ pub fn testnet_genesis(
 			}).collect::<Vec<_>>(),
 		}),
 		pallet_staking: Some(StakingConfig {
-			current_era: 0,
 			validator_count: initial_authorities.len() as u32 * 2,
 			minimum_validator_count: initial_authorities.len() as u32,
 			stakers: initial_authorities.iter().map(|x| {