[CIS-997] Fix over fetching previous messages

[nuno-vieira]

Description of the pull request

When getting the previous messages, we would make unnecessary requests. Now we only load the previous messages if there's more to fetch.

The only thing left to do is add test cases. I will do it once the implementation approach is approved.

[b-onc]

I think this approach is good 👍

[codecov-commenter]

Codecov Report

Merging #1110 (57f44c0) into main (75ad2de) will decrease coverage by 0.01%.
The diff coverage is 95.65%.

❗ Current head 57f44c0 differs from pull request most recent head 5b4d624. Consider uploading reports for the commit 5b4d624 to get more accurate results

@@            Coverage Diff             @@
##             main    #1110      +/-   ##
==========================================
- Coverage   91.38%   91.36%   -0.02%     
==========================================
  Files         219      219              
  Lines        9421     9432      +11     
==========================================
+ Hits         8609     8618       +9     
- Misses        812      814       +2     

Flag	Coverage Δ
llc-tests	91.21% <95.65%> (-0.02%)	⬇️
llc-tests-ios12	88.07% <95.65%> (-0.01%)	⬇️

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Impacted Files	Coverage Δ
Sources/StreamChat/Workers/ChannelUpdater.swift	97.93% <87.50%> (-2.07%)	⬇️
...trollers/ChannelController/ChannelController.swift	93.00% <100.00%> (+0.12%)	⬆️

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[evsaev]

Looks good 👍