Fix presence implementation according to reference one (ably-go)

[maratal]

Looks like a lot of outdated code in the presence, needs to be updated in according to reference implementation/specs.

Related issue - #1889

┆Issue is synchronized with this Jira Task by Unito

[sacOO7]

TODOs

1. Refactor onMessage implementation with relevant newerThan checks => https://github.com/ably/ably-go/blob/dd8236fc301eb078803e83efee7615f3ba133341/ably/realtime_presence.go#L291
2. Remove use of _syncSessionId. Currently, it's being used for marking members as updated. It causes field to be initialized in ARTPresenceMessage.m ( doesn't go as per spec ). We don't have syncSessionId in any other SDK's, seems this is there since very old spec. I would recommend to use beforeSyncMembers as a list instead.
3. Self-review ably-cocoa presence code with respect to ably-go implementation => https://github.com/ably/ably-go/blob/main/ably/realtime_presence.go

[maratal]

1. I think that checking nil in isNewerThan is okay and lets to avoid this check earlier. I've introduced additional check on whether the member exists prior to removing it. Also I think that the code duplication here can be also avoided, so I left cocoa implementation intact here, except renaming methods to something that I think makes more sense to reduce confusion.
2. I think that introducing additional array to track what was updated is unnecessary complication. Instead simple flag wasUpdated within a message can be used. I tried that (didn't rename the var though) and it works.
3. WIP

WDYT @sacOO7