sync: sync watch local version with shared version

[aym-v]

Implements the proposed solution to close #3666

Motivation

When a watch receiver is cloned, it does not use the latest version when calling changed(). It is therefore possible that if a receiver whose changed method has not been called is cloned, the call to changed() is not blocking.

See #3666 for more context

Solution

Implement a method that allows the user to synchronize the local version counter with the shared one

[Darksonn]

Having a method for doing something like this seems reasonable. I don't really like the name though.

[aym-v]

I admit that I have a hard time finding an ideal name as well.

Here are some proposals:
reconcile_version | reconcile_local_version | reconcile_version_counter
sync_version_counter | sync_local_version | synchronize_version

Maybe @pluyckx who reported the issue could help us on this?

[pluyckx]

I agree it is hard to find a good function name. If I look at the names you propose, I see you add some information about the object's implementation details.

Maybe a name like sync or update is better? In fact we are synchronizing the object's state. However, it is still not clear when/why you should call this function.

[MikailBag]

IMHO, this function is very specific and advanced, because it skips arbitrary amount of updates, and it is very easy to have a time-of-check-to-time-of-use bug (imagine that your task has been interrupted for an hour after the Receiver was creared, but before sync_version was called, highly likely applocation did not want to skip all this hour of updates, so it will behave incorrectly). That's why I'd give somewhat longer name, like skip_all_pending_notifications.

[pluyckx]

I think the name skip_all_pending_notifications is not perfect. As a user the word notifications confuses me. skip_all_pending_changes seems more appropriate. You wait for a change, not for a notification if you look at the API of a Watch. Not sure if the word all is necessary in the name.

The only question I have is, do we skip the changes or do we accept the last one? In the latter case then name accept_last_change is maybe a better option.

For me skip is a synonym for ignore in this case. So when calling borrow after calling skip... I expect the old value and not the latest sent to the Watch.