cluster: centralize removal from workers list.

Currently, cluster workers can be removed from the workers list in three
different places:
- In the exit event handler for the worker process.
- In the disconnect event handler of the worker process.
- In the disconnect event handler of the cluster master.

However, handles for a given worker are cleaned up only in one of these
places: in the cluster master's disconnect event handler.

Because these events happen asynchronously, it is possible that the
workers list is empty before we even clean up one handle. This makes
the assert that makes sure that no handle is left when the workers
list is empty fail.

This commit removes the worker from the cluster.workers list only when
the worker is dead _and_ disconnected, at which point we're sure that
its associated handles are cleaned up.

Fixes #8191 and #8192.

Reviewed-By: Fedor Indutny <fedor@indutny.com>