[Feature Request] Warn when runtime can't shutdown due left over NativeConnection

[mjameswh]

Describe the solution you'd like

By API contract, the user is responsible to close() NativeConnection objects they created explicitly (implicitly created connections are closed when its owner Worker is closed).

It is however too easy to forget doing so, and be left wondering why the process isn't completing as expected. With little work, we could emit warn messages when a NativeConnection is left opened for a few seconds after completion of the last associated Worker.