Add RevBank::Accounts::delete

[polyfloyd]

This makes deluser-like plugins possible.

[Juerd]

Thanks for your PR.

As discussed on IRC, I won't be merging it. Specifically, I think the plugin is not a good idea, because users are likely to expect a deluser command to delete a user account with everything associated with it. However, since the account name is the primary key used everywhere, and because a user's transaction log is a literal grep of the log file, deleting a user account from the accounts file and then using adduser again combined, is almost a noop. All it does is reset the datetime metadata.

There's a usecase for deleting accounts that have expired - those are unlikely to be used again, but I wouldn't expose the functionality to end users for that.

If you do choose to continue to use the deluser plugin, I'd strongly advise against the current implementation which uses RevBank::Accounts::update directly. Like the documentation for RevBank::Accounts describes, a transaction should be used instead, because otherwise there's a discrepancy between the ledger and the account balances.

I will cherry-pick your first commit, though, as it makes it easier to inactivate an account (that has expired) with the proper global lockfile.