Specify a minimum submission size that logs are required to support

@jsha raised this on the ct-policy [mailing list](https://groups.google.com/a/chromium.org/g/ct-policy/c/gjcdzPE1FlI/m/jSyukaZzCQAJ) , highlighting how TLS allows certificates up to 2^24 bytes (16.7 MB), while it's likely that logs may have a much smaller maximum client body size, or have implementations (e.g. MySQL) which have limits on maximum row size in some design.

Large certificates [totally exist](https://10000-sans.badssl.com/), and it should be better specified how to handle these.