ot::Time::GetDistantFuture() const method

Returns a new `Time` instance which is in distant future relative to current `Time` object. The distant future is the largest time that is ahead of `Time`. For any time `t`, if `(*this <= t)`, then `t GetGetDistantFuture()`, except for the ambiguous `t` value which is half range `(1 << 31)` apart. When comparing `GetDistantFuture()` with a time `t` the caller must ensure that `t` is already ahead of `*this`.