Fix auth genesis account number handling.

[SpicyLemon]

Description

This fixes the account keeper's InitGenesis to
a) Allow for missing account numbers.
b) Only change account numbers that are duplicated.
c) Accounts with duplicated account numbers get updated account numbers in a deterministic way (based on the order they were originally provided).

This also adds a simulation decoder for the newly added AccountNumberStoreKeyPrefix.

Prior to this PR, all account numbers in the provided accounts were rewritten to be sequential starting with 0. The list was sorted by account number before doing that, but if there was a missing account number, all account numbers larger than the missing one were reduced by 1. Also, since sort.Slice isn't guaranteed to be stable, if there were accounts with duplicate account numbers, the final "sorted" list was nondeterministic.

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Author Checklist

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• included the correct type prefix in the PR title
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• included the necessary unit and integration tests
• added a changelog entry to CHANGELOG.md
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• confirmed the correct type prefix in the PR title
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• reviewed state machine logic
• reviewed API design and naming
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• reviewed tests and test coverage
• manually tested (if applicable)

[SpicyLemon]

All unit tests passed locally (on the 3rd run).

1st run:

• /client failed: panic leveldb closed.
• /x/auth/client/testutil failed: couldn't find a transaction
• /x/auth/tx failed: queries were 1 tx short.
• x/feegrant/client/testutil couldn't find an account.

All of those are pretty usual on the first make test run.

2nd run:

• /client failed: empty proof.

3rd run:

• No failures.