Allow the MySQL command parser to process clauses in any order, fix #56.

Only the MySQL command is addressed in this patch, because the code
level approach is not safisfying me completely. It might be easier to
just bite the bullet and review all the optional clauses return values
rather than add a layer as this patch does.

The feature still is available for MySQL given this patch, so let's push
it, get feedback, then see about how to make the approach scale and
revise all the other commands.