fix(govdao): make Tiers private to prevent external modification

[moul]

memberstore.Tiers was a public variable (var Tiers TiersByName) that exposed the underlying *avl.Tree. External realms could call memberstore.Tiers.Set(...) to modify tier configurations,
escalating voting power or corrupting governance data.

h/t @jeronimoalbi for the report.

[Gno2D2]

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[MikaelVallenet]

lgtm, related to this i made a PR with linter that add rule that would trigger exporting global var -> #5068

[Davphla]

lgtm, it would be a good idea to verify any similar issues in other realms

[moul]

Exporting global vars make sense if they are safe objects.