Makes firestore a singleton and sets correct timestamp setting

[douweh]

I made the firestore instance a singleton on a newly created firestore service.
When asked for the instance I will also set the right settings getting rid of the deprecation warning in #43

[mikkopaderes]

Really appreciate for your time on taking a stab at this.

I'm not really liking the firestore.instance but I could see why you did it this way. I could probably refactor in the future should there be a cleaner solution that we can omit the .instance part. For now, this will do.

My only suggestion is maybe we can make this as not a computed property. We can assign the instance property in the init() hook. The settings will also be a standalone property so that other devs can override the service and provide their own settings if needed.

e.g.

export default Service.extend({
  ...

  settings: { timestampInSnapshots: true },

  init(...args) {
    this._super(...args);

    const firestore = this.get('firebase').firestore();

    firestore.settings(this.get('settings'));

    this.set('instance', firestore);
  }
});

[douweh]

I agree on both items:

• as a service needs to be an extension of Service, I could not see a way around this (the instance part) for now.
• I changed the code so the instance gets set on init (and settings are overridable)

I pushed a new commit.

Changes I think could be added but should not prevent this PR from merging:
-> get settings from ENV
-> Update your mock-cloud-firestore so it implements a settings method (tests were failing because of that, so I added the if-guard)