Feature mongo async

[Grubba27]

This PR is about updating the tutorial to start using the mongo API introduced in Meteor 2.8

@StorytellerCZ and @jankapunkt, do you guys have any good examples of using promises? I thought of something like with Tracker.autorun, as seen in this forum post, but I lack some context of Blaze.

Here is a sample of what I did before realizing that it does not accept promises as return value:

// with new Mongo Async API 
async tasks() {
    const instance = Template.instance();
    const hideCompleted = instance.state.get(HIDE_COMPLETED_STRING);

    const { pendingOnlyFilter, userFilter } = getTasksFilter();

    if (!isUserLogged()) {
      return [];
    }

    return await TasksCollection.find(
      hideCompleted ? pendingOnlyFilter : userFilter,
      {
        sort: { createdAt: -1 },
      }
    ).fetchAsync();
  }

// Old one

tasks() {
    const instance = Template.instance();
    const hideCompleted = instance.state.get(HIDE_COMPLETED_STRING);

    const { pendingOnlyFilter, userFilter } = getTasksFilter();

    if (!isUserLogged()) {
      return [];
    }

    return TasksCollection.find(
      hideCompleted ? pendingOnlyFilter : userFilter,
      {
        sort: { createdAt: -1 },
      }
    ).fetch();
  }

[jankapunkt]

Hey @Grubba27 we still have the issue, that Blaze helpers don't accept Promises. The linked issue is this one here: meteor/blaze#364

@StorytellerCZ from that perspective we should tackle the above mentioned issue asap, so this PR here can continue. WDYT?

[Grubba27]

I think is a must! What are the limitations for that issue to be solved? Do we have any possible implementations? Maybe go with something like what Svelte does with its async API but make it idiomatic for Blaze?

{#await promise}
	<p>...waiting</p>
{:then number}
	<p>The number is {number}</p>
{:catch error}
	<p style="color: red">{error.message}</p>
{/await}

[StorytellerCZ]

@jankapunkt I currently don't have usage of Blaze necessary to properly evaluate this PR, so I'll leave this to your discretion.