async state machines

[keppel]

this is important for a few reasons:

• allows lotion to be the glue between tendermint and a program written in any other language. this is the sort of thing where Node shines.
• allows a merkle-ized data store for the app state.

transactions will still be handled serially, they just don't have to mutate the state synchronously.

[keppel]

delaying async transaction handlers for as long as possible. probably will eventually support

app.use((state, tx)=>{
  return new Promise((resolve, reject)=> {
    // ...
  })
})

but let's keep tx handlers sync for a while.