extmsg: rework partial MID matching to favor current inbox

The current inbox is more important for partial Message-ID
matching, so we try harder on that to fix common errors before
moving onto other inboxes.  Then, prevent expensive scanning of
other inboxes by requiring a Message-ID length of at least 16
bytes.

Finally, we limit the overall partial responses to 200 when
scanning other inboxes to avoid excessive memory usage.