S3 Object ID Format

URI Path we write to:

/<key prefix>/<bucketname>/<dispersion prefix>/<dovecot internal path>

<key prefix> =	From URL config [optional; empty if not specified]
<bucketname> =	Either from URL hostname or URL “bucket=” query parameter
<dispersion prefix> =	From mail_location setting. Recommended value (see XXX) gives two levels of dispersion of the format: [0-9a-f]{2}/[0-9a-f]{3}
<dovecot internal path> =	Dovecot internal path to file. Example: $user/mailboxes/$mailboxguid/$messageguid

$user =	Dovecot unique username (installation defined)
$mailboxguid =	32 byte randomly generated UID defining a mailbox
$messageguid =	32 byte randomly generated UI defining a message blob