feat: Set last_used time upon account sign in

[aterga]

Motivation

According to the latest design, users who enable multiple accounts should see their accounts in the order in which they were last used. This requires that the order of usage is recorded on the Internet Identity backend, since otherwise the order cannot be known across the multiple devices that a user may have.

The storage as well as the canister API already supported last_used field, but it was not being populated.

To that end, this PR:

1. Prepares the code for recording last_used timestamps, slightly refactoring the storage code to bring about more clarity.
2. Starts populating the field.

Changes

• Add a new function with_account_mut that enables writing storage code for editing account details with a single account lookup (rather than one lookup to check if it exists, then another lookup to actually edit the account and record the changes).
• Add a new function set_account_last_used to Storage, call it from prepare_account_delegation.
• Call set_account_last_used account last used in prepare_account_delegation of the account management modules to populate the field when a delegation is created.

Tests

• Integration tests to check the new field is populated as expected.
• Unit test for the new storage method.
• Current unit and integration tests are enough to test that the refactor didn't change the expected behavior.

[sea-snake]

Very nice! Those additional tests clear up the questions I had.