index: support --max-size / publicinbox.indexMaxSize
In normal mail paths, we can rely on MTAs being configured with
reasonable limits in the -watch and -mda mail injection paths.
However, the MTA is bypassed in a git-only delivery path, a BOFH
could inject a large message and DoS users attempting to mirror
a public-inbox.
This doesn't protect unindexed WWW interfaces from Email::MIME
memory explosions on v1 inboxes. Probably nobody cares about
unindexed WWW interfaces anymore, especially now that Xapian is
optional for indexing.