Fix an issue with methods interacting with unblocking mechanisms

[nachocodoner]

OSS-352

Context: #13054 (comment)

Reproduction

Describe the next methods in the server.

let events = [];

Meteor.methods({
  async 'unblockedMethod'({ delay }) {
    events.push('unblock start');
    this.unblock();
    await Meteor._sleepForMs(delay);
    events.push('unblock end');
  },
  'blockingMethod'() {
    events.push('blockingMethod');
  },
});

Run on the client:

Meteor.callAsync("unblockedMethod", { delay: 200 }); // unblock + sleep for 20 milliseconds
Meteor.callAsync("blockingMethod"); // run straight + block

// assert 1: `events` should have `["unblock start", "blockingMethod"]` at this point as two methods are executed in parallel and the first one is unblocked. 
// assert 2: `events` should have added `["unblock end"]` at some point in the future as `unblockedMethod` finished

Check how the test is described on the PR. A failing scenario that reproduce the issue is depicted in the next picture. Warning logs Method took too long also showed up.

Fix

The fix relied on ensure the client queue triggers next method after the stub(simulation) finished for the previous one. This also fixed the logs.