cluster/node events (#1855)

[NaughtySora]

Description

Adding events for cluster client, some people were interested in it.
I have read the issue but didn't see any concrete approved suggestions so i come up with some.

for cluster:

1. connect - when cluster ready.
2. disconnect - when cluster is closed.
3. error (already exists).

for nodes:

1. node-connect - when tcp socket connected.
2. node-ready - when tcp socket are ready.
3. node-reconnecting - when tcp socket is reconnecting.
4. node-disconnect - when tcp socket disconnected.
5. node-error - when tcp socket errored.

Node events provide host/port to identify a particular node.

I would like to hear suggestions or opinion if this feature needed.

issue #1855

Checklist

• Does npm test pass with this change (including linting)?
  I tried to run tests via npm run build and npm test but it fails with timeout "before hook" after 30000ms after 1 hour of going a couple of time. But git checks are fine i guess.

• Is the new or changed code fully tested?
  I haven't seen any tests for emitting events, i have tested it only locally.

• Is a documentation update included (if this change modifies existing APIs, or introduces new ones)?
  Can't find where i can update desctiption about cluster events.
  I saw small section of client events on redis website.
  Can't update that.

[nkaradzhov]

@NaughtySora hi, sorry for big delay! This looks good! Could I ask you to:

1. Add some tests here that verify the events are properly fired? I have made a PR to your branch with a template of the test
2. Document the events in a new section called Events here. You can take inspiration from here

[NaughtySora]

Hi, thank you for the reply @nkaradzhov . I've added the test example from your PR, and I will proceed with the steps above .

[NaughtySora]

I added documentation inspired by standalone client.
Test is also included but I have 2 concerns.

1. I found only one way so far to get cluster's amount of nodes for testing. I'm looking for it in cluster._options, it can easily break encapsulation and cause troubles in the future.
2. I have tried to add negative routes for tests but faced the fact I have no idea how to force cluster node to reconnect in test framework. So the events 'error', 'node-error' and 'node-reconnecting' haven't tested yet.

[nkaradzhov]

I added documentation inspired by standalone client. Test is also included but I have 2 concerns.

1. I found only one way so far to get cluster's amount of nodes for testing. I'm looking for it in cluster._options, it can easily break encapsulation and cause troubles in the future.
2. I have tried to add negative routes for tests but faced the fact I have no idea how to force cluster node to reconnect in test framework. So the events 'error', 'node-error' and 'node-reconnecting' haven't tested yet.

1. Take a look here - you can specify how many masters/replicas you want your cluster. Essentially:

  testUtils.testWithCluster('foo', async cluster => {
    //tests
  }, {
    serverArguments: [],
    numberOfMasters: 3,
    numberOfReplicas: 3
  });

2. I think its fine to leave those untested for now

[NaughtySora]

I've found the way to make test more safe and changed docs a bit for more clear description.