Add indexes to multistep table

Thanks for the patch!

I started looking at the indexing, and I'm concerned it will affect I/O operations and disk space, so I came up with a few possible changes:

• Indexing the step column by itself provides no real gain, since you never query for that on its own. Instead, I combined it with the nid column, so that you can still get indexing, while taking advantage of MySQL's leftmost prefixing and avoid creating unnecessary indexes.
• The status index can be really cut down to just 1 character, since there are really only two options for that column: either submitted or unsubmitted.

I also added nid-step as a primary key, for what that's worth...

Can you please try the new patch and let me know if you see similar performance gains?

I added this to the new development snapshot. Thanks!