Bring Back shutdown_on_idle

[asevans48]

0.2.4

Windows 10 and Ubuntu 18.04 lts

Description

Shutdown on idle is now removed from the API. It is nice to have to be able to gracefully shutdown the system on a custom runtime. Can it be added back in?

[hawkw]

In tokio 0.2, tokio::spawn returns a JoinHandle. You can now use those JoinHandles to await the termination of spawned tasks in block_on, allowing more granular control over what futures keep the runtime running without requiring the runtime to track global idleness (which introduces overhead).

[carllerche]

#1879 is tracking a replacement. Until them what @hawkw said works.

[tyranron]

@carllerche #1879 is closed 😕
What about this one, can it be reconsidered?

The soultion provided by @hawkw doesn't work well in a common case, sometimes leading to very scary hacks when we need to pass those JoinHandles around via some global state.

The most common example when we need some notion of AsyncDrop, which we do today as Drop + tokio::spawn.