async_hooks: add copyHooks function

This commit introduces a copyHooks function that can be used by
storeActiveHooks and restoreActiveHooks to remove some code duplication.

PR-URL: https://github.com/nodejs/node/pull/19391


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Anatoli Papirovski <apapirovski@mac.com>