worker: improve integration with native addons

Native addons are now unloaded if all Environments referring to it have been cleaned up, except if it also loaded by the main Environment.

/cc @gabrielschulhof Who I remember had some suggestions for improving upon this approach

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• commit message follows commit guidelines