Werk #204: Reduced I/O on logins with access time recording or failed login counts

This change reduces the disk I/O produced by users accessing the multisite when the user access time recording is enabled. It should reduce I/O in installations with a large userbase.

In previous versions, these optional features were loading the whole user configuration just to read / change one attribute of one user, which is kept in a single file. This mechanism has been recoded to only use this single custom attribute file.

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