add fetched field to async tile data

[Bloomca]

Previously the data structure for async tile looked like the following:

{
  data: null,
  error: null,
  isPending: false,
}

The problem is that we can't keep null inside data (and all falsy values are hard to keep), so this PR moves responsibility of it to another field, fetched.
Now data structure looks like this:

{
  data: null,
  error: null,
  fetched: true,
  isPending: false,
}

So we can safely return null, and based on fetched decide whether it was done or not.

[coveralls]

Coverage increased (+0.3%) to 87.317% when pulling 63cfc45 on improvement/add-fetched-field-to-async-tile-data into 66bb431 on master.