Switch to Blockchain reactor v2

[liamsi]

The original blockchain reactor (v0) is a mess and hard to reason about (e.g. look at this: https://github.com/tendermint/tendermint/blob/ef56e6661121a7f8d054868689707cd817f27b24/blockchain/v0/reactor.go#L214-L366).

There has been quite some effort to refactor the current reactor and there are plans to switch to v2:
tendermint/tendermint#4595

The main blocker seems to be that the latest version isn't well tested. IMO we should start using the lastest version as soon as the integration tests work at least (see: tendermint/tendermint#4640). It will also make changes for immediate state execution simpler (ref #3).