Backport Future#timed as of cddd2b8

Original commit by @pchlupacek: Enhanced timed Future so it would not block the
Future`s execution thread pool, but instead will use scheduled executor to
notify timeout.

Changes to the original commit:

* no implicit `ScheduledExecutorService` (here: hard-wired)
* no overload for `Duration` (SIP-14 not available in 2.9.2)
* MiMa exception for adding a method to `Future`
give more threads to the timeout scheduler
Merge pull request #542 from larsrh/topic/future-backport

Backport `Future#timed`