cluster: reset handle index on close

Checklist

• tests and code linting passes
• a test and/or benchmark is included
• documentation is changed or added
• the commit message follows commit guidelines

Affected core subsystem(s)

Description of change

It allows reopening a server after it has been closed.

Fixes: https://github.com/nodejs/node/issues/6693.

/cc @cjihrig @bnoordhuis