Expose threadpool cleanup

[RReverser]

Exposing threadpool cleanup from libuv + adding missing API.

While proper cleanup is not critical in Node.js, where exiting process automatically kills all associated threads, it's important for environments like Stackblitz where otherwise spawned Worker will be stuck waiting on a condvar (on an atomic) forever in background even after the Node.js command has finished, leading to Worker leaks.

[toyobayashi]

Thank you for pointing out this, I added the public API uv_library_shutdown, which internally calls uv__threadpool_cleanup.

[RReverser]

Thank you for pointing out this, I added the public API uv_library_shutdown, which internally calls uv__threadpool_cleanup.

Hm I just added uv__threadpool_cleanup to EXPORTED_FUNCTIONS and called it from JS, that seemed to be enough. But I see libuv has uv_library_shutdown as well, so I guess makes sense to add it too.

[RReverser]

Could you do a patch or minor release please?

[toyobayashi]

Done.