Zulip: Enhance the Zulip presence engine to support 10K+ concurrent users in a realm

[timabbott]

The Zulip presence system will likely hit some performance issues for very large realm, where it won't make sense to send the presence data for every user in the realm to every other user.

236 will address some parts of this, but likely not everything.

[timabbott]

We made a ton of progress on this with the recent work on rerendering, etc. So far we've only tested the new system up to about 3K distinct users in a realm, mostly inactive. I think it'd be worth doing a test with 10K inactive users to see how well things work, and if it's acceptable. we can close this issue as complete!

[timabbott]

I'm going to close this, under the theory that we have no reason to believe our current strategy won't work at that scale, and we're unlikely to do this testing because of this issue (rather than because we have an organization at that size). chat.zulip.org will probably end up at 10K users before too long.