baseapp: do we need to store the header?

[ebuchman]

See the comment here: https://github.com/cosmos/cosmos-sdk/pull/521/files#diff-1a66280186fb23f75ab1d32920d81a1fR139

If not, we can clean up all that commented stuff and related logic in baseapp.

If so, either the AppHash will change with every block or we need to do surgery on the store interfaces to allow the header to be written to the db atomically with the other info.

Also, if we want the AppHash to not change every block, we need to deal with #520

[ValarDragon]

Closing per conversation that we don't need it. We haven't had a need for it in the last few months, and the header is already provided in begin blocker for applications.