Deprecate QFutureSynchronizer

QtFuture::whenAll creates a new QFuture that finishes when all the QFutures passed to it finishes. That's the same as QFutureSynchronizer does (minus the setCancelOnWait thing).

Further, QFutureSynchronizer<T> has the limitation that all QFutures must have the same T or it must be void for all of them. whenAll does not have this limitation so it's superior in this regard.

Additionally, the QFuture returned from whenAll can be used with QFutureWatcher to get signals.

So, other than cancelOnWait, it has been made redundant and we should deprecate it.
As a part of making sure it is exactly the same it would be a bonus if the implementation of QFutureSynchronizer was using QtFuture::whenAll.