Change GovInfoEvent's "unclaimed" field from Set to a Map

[lehins]

Description

@kderme, I this will be useful for db-sync to get access to the unclaimed amount right away, instead of forcing other to try and figure out unclaimed deposits from govActionId. Also the unclaimed amount is readily available in ledger anyways, so there is no point in discarding it.

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages.
  New section is never added with the code changes. (See RELEASING.md)
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated.
  If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• Code is formatted with fourmolu (use scripts/fourmolize.sh)
• Cabal files are formatted (use scripts/cabal-format.sh)
• hie.yaml has been updated (use scripts/gen-hie.sh)
• Self-reviewed the diff

[aniketd]

Looks good to me!

I have left a suggestion for the comment on the field.