Fix currently-checking-cache (#4410)

* alter currently-checking-set to launch work only on new candidates

* fmt

* fix compilation

* address review

* Introduce approvals cache test that ensures approval work is only triggered once for each Candidate Hash

* Fix formatting

* Address Feedback

* Move final message await into handle function

Co-authored-by: Chris Sosnin <chris125_@live.com>
Co-authored-by: Lldenaurois <Ljdenaurois@gmail.com>