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/executor/src/lib.rs

@@ -361,8 +361,6 @@ mod tests {
 				validator_count: 3,
 				minimum_validator_count: 0,
 				offline_slash: Perbill::zero(),
-				session_reward: Perbill::zero(),
-				current_session_reward: 0,
 				offline_slash_grace: 0,
 				invulnerables: vec![alice(), bob(), charlie()],
 			}),