Discard fetches when the active, logged-in account has changed

[gnprice]

When we go and fetch some data from the server, we should be sure to associate the resulting data with the right account, and not with some other of the user's accounts. In particular this should be the case even if the fetch took a while, and if in the interim the user went and switched to look at a different account (the new "active account".)

We'll need to solve this as part of #5005, where we'll potentially have several fetches going concurrently for several different accounts, and each will need to know which account the results should get stored under.

But pending that, in our current world where we have server data for just the one active account at a time, it'd be enough to discard the fetch's result (and ideally cancel the fetch itself, #4170) if the active account has changed by then.

Currently we do this in the event-queue long-polling loop in startEventPolling, which is the most important case, but not most other places -- notably, not in doInitialFetch registerAndStartPolling, or the various message-fetching actions in fetchActions.js.

Issues #4170 and #4659 cover parts of how we might do this.

Issue #3791 is basically an example symptom of this.

[gnprice]

We got a report today of what is probably this issue:
https://chat.zulip.org/#narrow/stream/48-mobile/topic/local.20cross-realm.20leakage.20issue/near/1444848

[chrisbobbe]

Any objection to changing the title from

Discard fetches when the active account has changed

to

Discard fetches when the active, logged-in account has changed

, since we probably want to cancel fetches when you log out the active account too?