src: use unique_ptr in platform implementation

Replace raw pointers in task queues with std::unique_ptr. This
makes ownership obvious.

PR-URL: https://github.com/nodejs/node/pull/16970


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>