Collation fetching fairness - handle group rotations

[tdimitrov]

Related to #1797 and follow up from #4880

On group rotation we could end up in the case where a fully backed candidate is provided by the old group for a claim queue slot falling into the next one. For example the claim queue holds paras [A, B, A] at RPX and two candidates for para A are provided. At RP X+1 a group rotation occurs. Now the claim queue has got [B, A, ...]. The second slot (A) is already claimed by the old validator group however the new one doesn't know about this. As a result we will end up with one additional collation.

This issue was initially reported by @eskimor: #4880 (comment)

[Oyase-shinobi]

Hello @tdimitrov
How are you doing
as I am a blockchain and web3 frontend developer, I would love to engage in your team
is there any opportunity to work with you?

[tdimitrov]

Hey @Oyase-shinobi. Thank you for your interest in polkadot-sdk!
I am already working on this particular issue but if you are interested in contributing to the project I'd suggest to have a look at the issues tagged with 'good first issue': C2-good-first-issue A task for a first time contributor to become familiar with the Polkadot-SDK. If you find something which you like write in the issue and ask to be assigned to it. You'll get help, feedback, code reviews etc.

Sounds like a plan?