Simplify Code for structure_dump

[maennchen]

Preparation for #419 (comment)

[josevalim]

Sorry for being a nuisance but this is also changing the implementation of how we collect versions and so on, and we don't need to do that, do we? Can we keep that functionality as is today?

[maennchen]

@josevalim I thought we have a good case for refactoring this part of the code since we're manually concating strings to build SQL. The change uses the dump command as well which supports dumping a single table.

I believe this is an improvement both in readability and the effective use of available tools.

If you do not agree, I will close this PR and start implementing the changes without touching any unrelated parts.

[josevalim]

I think not using string concatenation is a great feature but I wouldn't touch how we get/generate the versions. Let's not rock the boat too much. :)

[maennchen]

@josevalim Ok, then I'll just open a PR for the implementation of dump_cmd and close this one.