Adds a genesis parameter `initial_height` which specifies the initial block height, as well as ABCI `RequestInitChain.InitialHeight` to pass it to the ABCI application, and `State.InitialHeight` to keep track of the initial height throughout the code. Fixes#2543, based on [RFC-002](https://github.com/tendermint/spec/pull/119). Spec changes in https://github.com/tendermint/spec/pull/135.
Reorganizes the Protobuf schemas. It is mostly bikeshedding, so if something is contentious or causes a lot of extra work then I'm fine with reverting. Some Protobuf and Go import paths will change.
* Move `abci/types/types.proto` to `abci/types.proto`.
* Move `crypto/keys/types.proto` and `crypto/merkle/types.proto` to `crypto/keys.proto` and `crypto/proof.proto`.
* Drop the use of `msgs` in filenames, as "message" is a very overloaded term (all Protobuf types are messages, and we also have `message Message`). Use `types.proto` as a catch-all, and otherwise name files by conceptual grouping instead of message kind.
William Banfield
Callum Waters
Marko
Sam Kleinman
Erik Grinaker