Refactor `sandboxGlobals` options into `buildSandboxGlobals`

[rwjblue]

Now that we are not sharing a single sandbox context for all visits, we will need to change the API to provide custom globals from a single stable object with properties to be assigned into the VM sandbox to a builder function that will return a new object for each request.

Specifically, we will move from:

let app = new FastBoot({
  distPath: 'path/to/dist',
  sandboxGlobals: { someProperty: whateverHere },
});

To:

let app = new FastBoot({
  distPath: 'path/to/dist',
  buildSandboxGlobals() { 
    return { someProperty: whateverHere };
  },
});