[LTS patch] WebHost.Run() completes before ApplicationStopping

[muratg]

Original issue: #812, fixed in 1.1.0-preview1.

ApplicationStopping exits before ApplicationStopped, preventing app to take any measures it may need before getting stopped. Fix is simple and low risk.

We'd like to bring this in an upcoming LTS-patch release.

cc @Eilon @DamianEdwards @davidfowl

[Eilon]

This patch is approved, please ensure it is merged into the correct branch and building as part of the patch train.

[JunTaoLuo]

This is completed in da5ea83.