Tx interface

[ebuchman]

Wrote about recent Tx structure discussion here: #669

I think this is all great, but in looking at the interplay with baseapp, I'm not sure we want to thread a struct through that. It's really just the AnteHandler that sees the transactions, and I can forsee many forms of ante handler (we have one, ethermint has another, surely there's more).

So instead, I propose we have type Tx interface{} (an empty interface), since baseapp doesn't really need anything from a Tx. Then in ante handlers, we'd do the type assertions. I suspect this pattern might emerge anyways, so maybe it's best to make it explicit up front. The AnteHandler can also include the TxDecoder and SignBytes functions, since those are tightly coupled to the relationship between bytes and Tx Structure.

In sum, AnteHandler is responsible for the map between an empty Tx interface and the underlying Tx and Serialization structures. StdTx is our primary (sdk default) way of making a Tx, and its AnteHandler is auth.

I think the baseapp is a really useful abstraction over ABCI that can exist more independently of the Tx specifics (possibly MultiStore too ...).

[cwgoes]

Per SDK design meeting, punted for now.

[ebuchman]

This can be closed. Still need to reduce baseapp dependency on auth - noted in #1579