src: do not persist fs_poll handle in stat_watcher

Instead of relying on garbage collection to close the handle,
manage its state more explicitly.

PR-URL: https://github.com/nodejs/node/pull/21093
Fixes: https://github.com/nodejs/node/issues/18190
Refs: https://github.com/nodejs/node/pull/18307


Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>