newswww: use ->ALL to avoid O(n) inbox scan

newswww: use ->ALL to avoid O(n) inbox scan

We can avoid doing a Message-ID lookup on every single inbox
by using ->ALL to scan its over.sqlite3 DB.  This mimics NNTP
behavior and picks the first message indexed, though redirecting
to /all/$MESSAGE_ID/ could be done.

With the current lore.kernel.org set of inboxes (~140), this
provides a 10-40% speedup depending on inbox ordering.