[Patch] Address DAS todos

[Brent Daniel <br...@gmail.com>]

This patch addresses some DAS code "TODO" statements. Except for the first
two, these are mostly no-ops.

1) Refactor ApplyChangesCommand to be part of a single Command heirarchy
2) Removed the instanceof hack in ChangeOperation.java
3) Removed TODO from SDODataTypes -- No longer relevant
4) Removed TODO from CompoundKeyTests -- Statement no longer correct
5) Removed old, dead, commented code from the TopDown test class.
6) Removed TODO in InsertList.java -- Verified that the null check is OK
here and added a comment
7) Removed TODO statement  from CommandGroupImpl -- Nothing to do here, and
no longer relevant