Support empty meta-blocks with optional ignored metadata.

This is a partially backward incompatible format change,
that makes previously valid brotli streams that contain
larger than 16MB meta-blocks invalid.

The impact of this should be minimal, since the 'bro'
command-line tool does not create larger than 2MB
meta-blocks, so the only streams this change could
break are those created by a custom brotli encoder.

This commit contains only the specification update,
implementation in the decoder and encoder will
follow in later commits.