Optimize marking views as read/unread using the most efficient server endpoint for our state

[timabbott]

Similar to zulip/zulip#29521, we can choose between the two different valid ways to ask the server to mark a given view as unread based on whether we can prove a smallish number of message IDs that we have cached is complete or not.

[gnprice]

Thanks!

Chat thread with a bit more context. This should mean a noticeable speedup when marking a conversation (other than a very long one) as read or unread.

We don't currently offer marking as unread — that's tracked as #131. The biggest value for this optimization is in marking as unread, because the speed of marking as read is invisible to the user: it's one of the few places where the Zulip clients show a change immediately rather than waiting for the server to confirm it. So this will be most useful after #131.