[#316] Remove proposal limit and use a linked list for sorting

[rinn7e]

Description

Problem: There is currently a limit on the number of concurrent ongoing proposals, due to the fact that proposals need to be sorted (for flush) and that this is done using a set, which cannot grow indefinitely without causing gas cost problems.

Solution: Remove proposal_key_list_sort_by_level and use a doubly linked list instead to store the proposals in order.

Related issue(s)

Resolves #316

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Related changes (conditional)

• Tests

  • [x] If I added new functionality, I added tests covering it.
  • [x] If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.

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  • [x] I checked whether I should update the docs and did so if necessary:
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  • Haddock

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[rinn7e]

@pasqu4le Took me a while to finish fixing the tests. I also fix some bugs along the way.

This PR is ready now, hope to get your review on this.