[FIX] Apps-Engine's scheduler failing to update run tasks

[thassiov]

Proposed changes (including videos or screenshots)

Agenda, the library that manages scheduling, depended on setting a job property named nextRunAt as undefined to signal whether it should be run on schedule or not. Rocket.Chat's current Mongo driver ignores undefined values when updating documents and this was causing jobs to never stop running as Agenda couldn't clear that property (set them as undefined).
This updates Rocket.Chat's dependency on Agenda.js to point to a fork that fixes the problem.

Issue(s)

Steps to test or reproduce

Starting from Rocket.Chat@3.17.0:

Install this example app. At startup, it creates a job that should run only once 10 seconds after it is registered.
schedule-once_0.0.1.zip

It runs every 5 seconds (minimum amount of time allowed by Agenda) indefinitely.

Further comments

First iteration:

Make mongo driver's configuration about bson types explicit so undefined fields are recognized and not dropped by the query parser. The issue started happening after Meteor's 2.2 update.

Agenda, the Apps Engine's dependency that manages apps scheduling, needs this feature as it depends on setting certain values as undefined when updating documents. For the case of tasks that run only once, the property nextRunAt, that determines if a job must be run in the future, is never set to undefined, thus making the scheduler run it indefinitely.

Somewhere down the dependency tree some function is ignoring or changing this value. As of right now we couldn't identify where, but it prevented updates with fields having undefined as their values. The problem is solved when we set this mongodb property explicitly.

Related documentation:

Node.js MongoDB Driver API: https://mongodb.github.io/node-mongodb-native/3.6/api/Db.html

Second iteration (current):

Fix agenda.js itself and update Rocket.Chat's dependency to the fixed version.

[sampaiodiego]

this change is breaking some endpoints, can you please take a look?

Approach has changed since review