[Feature] Future.await with timeout

[magicprinc]

Future has await() method without timeout

It is easy to implement and nice to have await(timeout,TimeUnit) method

[vietj]

that is not needed since it possible to use timeout on a future, i.e await(future.timeout(timeout)), instead we should document this usage in the documentation.

[magicprinc]

@vietj Then #4958
NullPointerException is real (see test)
And quick path for already completed futures is also a good idea