Async: Add future<>::fallback_to

[viboes]

Quite often the user as a fall back value that should be used when the future has an exception.

This factory creates a new future that will fall back to the parameter if the source future will be ready with an exception. The following

  f.fallback_to(v);

is a shortcut for

  f.then([](future<T> f) {
    return f.get_or(v);
  });

[viboes]

Committed revision [84719].

[viboes]

Changeset [85200]