Move from Guava ListenableFuture to Java 8 CompletableFuture

Guava Futures and ListenableFuture was used for the async commit changes. Now that we have switched to java 8, should look in to using CompletableFuture. Also Futures in marked @Beta in guava, so it could change in incompatible ways or be dropped in the future.

[jkosh44]

@keith-turner The indirection worked, just pushed the fix.

[jwonders]

The purpose of AsyncCommitResult is to capture the result of the async operation. Terminal steps would be able to create a concrete value of this result class because they are aware of successful completion or errors. Intermediate steps would return a future based on additional composed async steps.

In the code I had posted, the finishCommit method is a terminal step that returns the committed() status upon successful completion. The deleteLocks is an intermediate step that does its work then composes the result with the finishCommit step. Some steps might return immediately upon exceptional conditions and perform composition on normal conditions.