unread: Fix mark all as read when oldest unread message is muted.
unread: Fix mark all as read when oldest unread message is muted.

As detailed in the comment, the new "mark all as read" bankrupcty code
path would fail to mark as read muted stream messages that are older
than the oldest unread non-muted message.

This was because of the semantics of the `first_unread` anchor value
being actually "fire unmuted unread".

We can just change this to "oldest", because we're already using an
`is:unread` for the narrow query; doing so likely is a small
optimization, since we save the query to compute the first unread.

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