Spring SPR-15413: Add Kotlin coroutines for ListenableFuture

Prior to this commit there was no support for converting Kotlin coroutines to/from ListenableFuture. Following that change a number of functions will be available for such conversion.

Specifically these functions are introduced:

// for converting suspending lambda to ListenableFuture fun <T> listenableFuture(context: CoroutineContext = CommonPool, block: suspend () -> T): ListenableFuture<T>
// for converting Deffered into ListenableFuture fun <T> Deferred<T>.asListenableFuture(): ListenableFuture<T>
// for converting ListenableFuture into suspending function suspend fun <T> ListenableFuture<T>.await(): T

The code in this PR draws heavily on the code from https://github.com/Kotlin/kotlinx.coroutines/blob/master/kotlinx-coroutines-jdk8/src/main/kotlin/kotlinx/coroutines/experimental/future/Future.kt. This is why Roman Elizarov is also an author.

Comment From: enleur

Just to have a reference to the issue https://jira.spring.io/browse/SPR-15413

Comment From: snicoll

@sdeleuze can you please check if this is still applicable?

Comment From: poutsma

After team discussion, we have decided not to invest in ListenableFuture. It can easily be converted into a CompletableFuture, which has Kotlin support through kotlinx-coroutines-jdk8.