Support java.util.Optional for @MVC named value args

After this change, java.util.Optional is supported with @RequestParam,
@RequestHeader, and @MatrixVariable arguments in Java 8. When Optional
is used the required flag is effectively ignored.

Issue: SPR-11829
Created SettableListenableFuture with tests

A SettableListenableFuture implementation of Spring's ListenableFuture
The class is inspired by Google Guava’s
com.google.common.util.concurrent.SettableFuture, but this
implementation uses ReentrantReadWriteLock and CountDownLatch
internally to handle thread synchronization.

Issue: SPR-11614
Revise SettableListenableFuture implementation

This change modifies the SettableListenableFuture implementation to use
internally a ListenableFutureTask created with a "settable" Callable.

Issue: SPR-11614
Merge pull request #504 from matsev/SPR-11614
Add suppress warning in SettableListenableFuture