[lib][native][web] Fix bug where no messages are loaded if most recent 20 are reactions

Summary:
This resolves ENG-3851. There are two issues here:

1. oldestMessageServerID is checking messageListData, which does not include reactions by themselves. If new results are all reactions, messageListData will not update, and oldestMessageServerID will stay the same.
2. The code for setting loadingFromScroll relies on messageListData changing, which (as mentioned above) is no longer guaranteed if returned messages are reactions.

Test Plan: I tested this on both web and native by creating over 20 reactions in my local environment. That allowed me to repro the issue after an initial load on web or a log-in on native. I was able to confirm the issue was resolved with this diff

Reviewers: atul, tomek, ginsu

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D7746