`Events::oldest_event_count` and `Events::oldest_id` don't make the difference to each other clear enough

[urben1680]

I am not certain if this is a docs issue, but I found this by looking at the source, not by encountering some bug. That is why for now I focused on how the API appears while there are more issues.

The methods Events::oldest_event_count and Events::oldest_id seem to do the same thing when looked at the docs or the source.

Also the API itself is confusing. Instead of returning usize it should return an Option<EventId<E>> because no oldest event might be available currently if the sequences events_b and events_a are empty.

As a side note, the implementation of oldest_event_count looks like that:

/// Returns the index of the oldest event stored in the event buffer.
pub fn oldest_event_count(&self) -> usize {
    self.events_a
        .start_event_count
        .min(self.events_b.start_event_count)
}

When it should be clear that self.events_b as the more recent sequence cannot contain the oldest count.

• oldest_event_count was added by [Merged by Bors] - Warning message for missing events #5730
• oldest_id was added by [Merged by Bors] - Rework manual event iterator so we can actually name the type #5735

[urben1680]

Tagging @Aceeri as the author of both methods. 👋

[Aceeri]

Ya oldest_id should just be renamed to oldest_event_count and it's implementation is generally better. Probably just missed this during merging + the reverting of the warning message.