faster
- Stop adding pm chat messages received to unread messages. This is in addition to [8311], but
works also when chat win is already opened now.
- Automatically remove unread_message lines for messages older than a month, so we will clear this
table for everybody, and keep it quite clear. Specially because messages can still be blocked here
if it's from someone not in the roster and we don't open it during the session. This can be long
at first connection with this version, because we delete a lot of lines (nearly one per PMs you
received since a long time). Could also be a very little longer during one month.
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