Only maintain at most 1 `UnlockChunk` per era (#10670)

* Only maintain at most 1 `UnlockChunk` per era

* Bound `unlocking`

* Run cargo +nightly-2021-10-29 fmt

* Make benchmarks stuff compile

* Update frame/staking/src/lib.rs

Co-authored-by: Kian Paimani <[email protected]>

* Remove DerefMut; Implement neccesary methods directly

* Doc comments for new BoundedVec methods

* Fix benchmarks

* wip bonded_vec macro

* Correct rust doc

* Apply suggestions from code review

Co-authored-by: Kian Paimani <[email protected]>

* Update staking::Config impls

* Add MaxUnlockingChunks to more places

* Use defensive saturating add

* FMT

Co-authored-by: Kian Paimani <[email protected]>