Optimize user list handling to improve startup and in-app performance

[preetmishra]

This is not as apparent with servers that have a limited number of users. However, with CZO, which currently has ~16000 users, the performance hit is quite evident.

We should rework the way the user list loading/updating is handled currently to improve the app startup time and address other potential performance-related issues (e.g. user search reset in #700).

Related conversation link: https://chat.zulip.org/#narrow/stream/206-zulip-terminal/topic/Startup.20time/near/675442.