Refactor `Uni.awaitSuspending`

[hantsy]

Follow the existing extension modules from https://github.com/Kotlin/kotlinx.coroutines/tree/master/reactive, add Uni.awaitSingle() and Uni.awaitSingleOrNull instead, the naming is more clear and easy migrated from other ReactiveStreams/JDK9 Flow impls.

[heubeck]

lets follow up on this discussion in #1325