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* remove outdated comment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/frame/session/src/mock.rs

@@ -92,6 +92,7 @@ impl SessionHandler<u64> for TestSessionHandler {
 pub struct TestSessionManager;
 impl SessionManager<u64> for TestSessionManager {
 	fn end_session(_: SessionIndex) {}
+	fn start_session(_: SessionIndex) {}
 	fn new_session(_: SessionIndex) -> Option<Vec<u64>> {
 		if !TEST_SESSION_CHANGED.with(|l| *l.borrow()) {
 			VALIDATORS.with(|v| {
@@ -112,6 +113,7 @@ impl SessionManager<u64> for TestSessionManager {
 #[cfg(feature = "historical")]
 impl crate::historical::SessionManager<u64, u64> for TestSessionManager {
 	fn end_session(_: SessionIndex) {}
+	fn start_session(_: SessionIndex) {}
 	fn new_session(new_index: SessionIndex)
 		-> Option<Vec<(u64, u64)>>
 	{