genesis: add support for arbitrary initial height (#5191)

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](tendermint/spec#119). Spec changes in tendermint/spec#135.