Add id's to various elements in protocol.sgml

When working with the Frontend/Backend Protocol implementation in Npgsql
and discussing things with the team, I often struggle with the fact that
you can't set deep links to individual message formats in the somewhat
lengthy html docs pages.

The attached patch adds id's to various elements in protocol.sgml to
make them more accesssible via the public html documentation interface.

I've tried to follow the style that was already there in a few of the
elements.

Do you consider this useful and worth merging?

Is there anything I can improve?

Best Regards,

Brar