baseapp does not check block ordering

[ValarDragon]

Baseapp currently gets the header height from the ABCI begin block request. It does not validate anywhere that this header height is non-negative, or that it equals previous height plus one, however this is an assumption we make within in the state machine. For improved safety as we start trying new underlying consensus algorithms, we should likely enforce this check within baseapp. (I did not check whether Tendermint enforces this, but I would assume that it does)

(Note that if we do things like proof of work with the SDK, handling histories and etc. would likely be the responsibility of Tendermint)