ADR/Baseapp++: prepare and process proposal design

Write and merge an ADR with the first two phases of abci++, [prepare](https://github.com/tendermint/tendermint/blob/master/spec/abci%2B%2B/abci++_methods.md#prepareproposal) & [process proposal](https://github.com/tendermint/tendermint/blob/master/spec/abci%2B%2B/abci++_methods.md#processproposal). 

