Overview of the Issue

If a table is in the middle of a schema migration it's possible for a scattered SELECT * query to return an inconsistent schema depending on the state of each tablet. When aggregating results we should ensure that each result contains the same set of fields and return an error if not.

Caveat: I'm not positive this is not already handled but based on commentary in this comment I don't believe it is.

Reproduction Steps

1. Begin making a schema change
2. Issue a scatter select * before the schema is applied to all shards