domain: use strong reference to domain while active

When an uncaught exception is thrown inside a domain, the domain is
removed from the stack as of 43a51708.
This means that it might not be kept alive as an object anymore,
and may be garbage collected before the `after()` hook can run,
which tries to exit it as well.

Resolve that by making references to the domain strong while it is
active.

Fixes: https://github.com/nodejs/node/issues/28275

PR-URL: https://github.com/nodejs/node/pull/28313


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Vladimir de Turckheim <vlad2t@hotmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>