Add flag to disable synchronous IO

[davidfowl]

Because blocking threads is bad so we want a way to turn this off synchronous IO by default to let you shoot yourself in the foot if you want this behavior.

[benaadams]

throw new NotWiseException()/throw new NotScalableException()?

[Drawaes]

Throw new Bang (Target.Foot)

[DamianEdwards]

@davidfowl you planning to get this added this week?

[davidfowl]

@DamianEdwards I'm not sure we can do this for preview. There's too much churn and too many other important things to do in Kestrel right now (like stabilize the build).