[#290] Check proposal really exist in existence check

[sras]

Description

Problem: It seems that we don't actually check the 'proposal_key_list_sort_by_date' structure for the proposal existence check. And we should since we only remove proposals from this set during proposal flush/delete operations.

Solution: In additon to the check in proposals bigmap, check if 'proposal_key_list_sort_by_date' also contains the proposal before returning success. Add some tests that it works as expected.

Related issue(s)

Resolves #290

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[sras]

One last comment; it seem that the commit message is missing the Problem/Solution, but those are in the PR description instead.

Oh I had fixed it in a local commit, but missed to push. Have pushed it now.