Support downgrading to a long-lived event queue

[chrisbobbe]

@timabbott mentions on czo that zulip/zulip#12926 is the PR thread tracking a server feature for the ability to downgrade to a long-lived event queue that ignores high-traffic events.

This would cover #1804 by making it unnecessary to fetch all users at startup; long initial load times are a common problem for users (see also the czo link, above), but I've made a new issue because there are separate benefits and implementation details to consider.

Unrecognized events will be ignored all the way back to mobile commit d2836e1, 2017-03-11, v1.0.11 (see discussion). Progress on feature detection via version number parsing is underway at #3839; see also the discussion about a feature level integer in, e.g., the /server_settings response, which we'll want to switch to when possible.

[gnprice]

Unrecognized events will be ignored all the way back to mobile commit d2836e1, 2017-03-11, v1.0.11 (see discussion).

It looks like that reference was about events with the update_message type, but with a new field.

We do ignore events of unexpected type (in responseToActions in eventActions.js), though, and I think we have for a long time. Fortunately also it doesn't matter how long we have (or even if we already do or not), because this will be a feature that new clients opt into.

[gnprice]

This blocks on #4841 and #3408.

Then when the blockers are cleared, the work of this issue will be to a large extent on the server, as well as in the client.