Add batching to fast-unstake pallet (#12394)

* implement a brand new batch with all tests passing.

* fix benchmarks as well

* make benchmarks more or less work

* fix migration

* add some testing

* Update frame/fast-unstake/src/benchmarking.rs

Co-authored-by: Roman Useinov <roman.useinov@gmail.com>

* review comments

* some fixes

* fix review comments

* fix build

* fmt

* fix benchmarks

* fmt

* update

Co-authored-by: Roman Useinov <roman.useinov@gmail.com>