cluster: return worker reference from disconnect()

Checklist

• make -j8 test (UNIX), or vcbuild test nosign (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

Affected core subsystem(s)

cluster

Description of change

Changes disconnect() to return a refererence to the worker. This will enable method chaining such as worker.disconnect().once('disconnect', doThis);