src,lib: the handle keeps loop alive in cluster rr mode

[theanarkh]

The handle does not keep loop alive in cluster rr mode, the code is as follows.

const http = require('http');
const cluster = require('cluster');
cluster.schedulingPolicy = cluster.SCHED_RR;

if (cluster.isMaster) {
    cluster.fork();
} else {
    http.createServer((req, res) => {
        res.end('ok');
    }).listen(8888, () => {
        process.channel.unref();
    });
}

So try to fix this issue.

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

[bnoordhuis]

LGTM (improves on the existing state of things) but see comment.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/48959/

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/48961/

[nodejs-github-bot]

Landed in 7895484