ref: use new prepare code and remove some old

[jordanrfrazier]

This removes the column_behavior in favor of the simpler, newer prepare_batch logic. This also fixes the "floats not allowed as timestamps" behavior, since the new logic handles that.

Closes #776

[bjchambers]

If this passes all the tests... that's awesome. Deletes a bunch of old code and fixes bugs. I think the one thing that we maybe lose is the ability to drop/evolve columns and such -- but I think that can be added to the existing prepare batch -- if we know the "expected" schema, we can just omit/evolve columns based on the file schema and expected schema. Definitely simpler!