message_fetch: Support storing multiple contiguous blocks of history

[timabbott]

This is a feature that likely only makes sense to do once we've completed #15131, but might be a design consideration for how we complete that effort.

Ideally, the message_list_data data structures would support caching more than one contiguous block of message history. The obvious use case is when we're loading the home_msg_list data:

• We want to fetch message history around the first_unread message, because that's where we'll land users when they click around the UI at places with unread messages.
• We want to also fetch message history around the latest messages, because that is where we'll land users when they click around the UI on places without unread messages, and is also what we need to display Recent Topics data.

We ideally want this feature for narrowed message lists as well, at least once we've done #15131, since that would support rendering message history without going to the server in more cases.

I'm not sure what the right way to structure this is; one idea would be to support multiple message_list_data objects for a given filter, one for each contiguous segment; then the FetchStatus logic (etc.) would all work naturally, and we'd just merge the objects once their message ID ranges overlap. Certainly that version would only make sense to consider once we've finished #15131.

@ryanreh99 @amanagr FYI.

[zulipbot]

Hello @zulip/server-message-view members, this issue was labeled with the "area: message view" label, so you may want to check it out!

[timabbott]

#23812 has a writeup of important preparatory work towards this goal.