Better for service discovery

[hustxiaoc]

For some situation, the mongo server host is dynamic and we need to create a new mongodb client when the current client is disconnected, so it's much better if we can call setClient multiple times.

const createMongoClient = async () => {

  const host = await service.discover('mongodb.host');

  const uri = `mongodb://${host.uri}/test`;

  const mongoClient: MongoClient = await MongoClient.connect(uri, {
    autoReconnect: false,
  });

  client.setClient(mongoClient);

  client.once('disconnected', async () => {
    mongoClient.close();

    // @ts-ignore   we need to get rid of this
    client.client = null;

    while(true) {
      await sleep(3 * 1e3);
      try {
        if (client.readyState === mongoose.STATES.disconnected) {
          await createMongoClient();
        }  
        break;
      } catch (err) {
        console.error(err);
      }
    }        
  });
}

await createMongoClient();

[vkarpov15]

Supporting this sort of functionality is risky, because we don't have a good way to cancel operations in flight. For example, suppose you're in the middle of a populate() when you call setClient() - main query executes on the first MongoClient, 2nd query may execute on a separate client connected to a separate database.

That being said, I don't see any reason why we can't merge this. If you choose to use this behavior, just choose to be aware of the risks: calling setClient() does not cancel any operations that are in flight.