Be consistent about threaded vs basic scheduler in v0.3

[Darksonn]

Currently Runtime::new() and #[tokio::main] will make their choice about the kind of runtime by inspecting the rt-threaded flag. This should be changed in Tokio v0.3, and code uses #[tokio::main] without rt-threaded should probably error with a suggestion to enable the feature or to use the basic scheduler.

We should also overhaul the runtime builder.

[carllerche]

Superseded by #2698

[benesch]

Superseded by #2698

#2698 is this very issue :D. For posterity, I believe the correct superseding issue is #2720.