cluster: move handle tracking out of utils

internal/cluster/utils.js exported a handles object, which was
used in a test. That test, test-cluster-disconnect-handles.js,
was removed in https://github.com/nodejs/node/pull/12495. This
commit moves the handles object to the only file in the codebase
that still uses it.

PR-URL: https://github.com/nodejs/node/pull/23131


Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>