worker: use rwlock for sibling group

Since it is much more common to send messages than to add or
remove ports from a sibling group, using a rwlock is appropriate here.

Refs: https://github.com/nodejs/node/issues/38780#issuecomment-846548949

PR-URL: https://github.com/nodejs/node/pull/38783


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>