Real time support

[ClemensLey]

This PR adds real time support to the Bitcoin Computer. It adds a new function subscribe to the Computer class that provides real time updates.

async subscribe(id: string, onMessage: (rev: string) => void, onError?: (error: Event) => void)

Specifically, the function takes two arguments, and object id and a callback function that is called every time a function is called on the object. For example:

const a = await computer.new(C)
close = await computer.subscribe(a._id, (rev) => {
  console.log(`Update to object with id ${a._id}: the latest revision is ${rev}`)
})
await a.inc(1)
await a.inc(1)

// logs "updated on object with id ..." twice

Behind the scenes we use server side events (SSEs) which is a minimal high performance technology for server side pushs.