feat: add thread-safe Scheduler for spawn using ngx_notify

schedule() can now be called from any thread and will move tasks to the event loop
thread using ngx_notify (ngx_event_actions.notify). This enables receiving I/O
notification from "sidecar runtimes" like async-compat, and requires less unsafe.

The async example has been rewritten to use async_::spawn, demonstrating usage of
reqwest and hyper clients wrapped in Compat to provide a tokio runtime environment while
using the async_ Scheduler as executor.

Author: pschyska