imap: break giant inboxes into sub-inboxes of 50K messages
This limit on mailbox size should keep users of tools like
mbsync (isync) and offlineimap happy, since typical filesystems
struggle with giant Maildirs.
I chose 50K since it's a bit more than what LKML typically sees
in a month and still manages to give acceptable performance on
my ancient Centrino laptop.
There were also no responses to my original proposal at:
<https://public-inbox.org/meta/
20200519090000.GA24273@dcvr/>
so no objections, either :>