Staking rate targeting and specific rewards. (#2882)

* PNPoS implementation

* wip: change staking api

* code readibility

* fix overflow

* comment

* license

* doc

* reorganize a bit

* rename to proper english + doc

* address comments

* refactor unused mock

* fix add_point

* update tests

* add not equalize to ci

* Revert "add not equalize to ci"

This reverts commit 1c61c25874b04b1cdfadd7d353b75a189a59ad35.

* bring test back

* update locks

* fix genesis config

* add authorship event handler + test

* uncouple timestamp from staking

* use on finalize instead

* remove todo

* simplify mock

* address comment

* doc

* merge test

* fmt

* remove todo todo

* move add_reward_point to regular function

* doc

* doc

* increase version

* doc and fmt

* Update srml/staking/src/inflation.rs

Co-Authored-By: Bastian Köcher <bkchr@users.noreply.github.com>

* Fix some doc typos

substrate/node/cli/src/chain_spec.rs

@@ -121,8 +121,6 @@ fn staging_testnet_config_genesis() -> GenesisConfig {
 		staking: Some(StakingConfig {
 			current_era: 0,
 			offline_slash: Perbill::from_parts(1_000_000),
-			session_reward: Perbill::from_parts(2_065),
-			current_session_reward: 0,
 			validator_count: 7,
 			offline_slash_grace: 4,
 			minimum_validator_count: 4,
@@ -259,8 +257,6 @@ pub fn testnet_genesis(
 			minimum_validator_count: 1,
 			validator_count: 2,
 			offline_slash: Perbill::zero(),
-			session_reward: Perbill::zero(),
-			current_session_reward: 0,
 			offline_slash_grace: 0,
 			stakers: initial_authorities.iter().map(|x| (x.0.clone(), x.1.clone(), STASH, StakerStatus::Validator)).collect(),
 			invulnerables: initial_authorities.iter().map(|x| x.0.clone()).collect(),