startEventPolling [nfc]: Be clearer that this is a per-account action

In a multi-account, post-zulip#5005 world, we're likely to want a live
event queue for all the logged-in accounts. So we'll want this
function to start polling for "this account", not for the global
"active account". The action is already typed as a `ThunkAction`,
not a `GlobalThunkAction`; that's a good start.

The `auth` object we're working with after the code change is the
same one as before, we just get it more directly. (`tryGetAuth` uses
`getAccount`, which, pending zulip#5005, assumes "the account" is the
active account; see the implementation comment in `getAccount`.)
And, with zulip#5005, it will naturally change to be the Auth object for
"this account", the one the caller cares about.

Also expand a few comments and clarify that they only apply in the
pre-zulip#5005 world, and add TODOs to remove them when we complete zulip#5005
so we don't forget.

src/events/eventActions.js

@@ -1,12 +1,11 @@
 /* @flow strict-local */
 import type { GeneralEvent, ThunkAction } from '../types';
-import { assumeSecretlyGlobalState } from '../reduxTypes';
 import * as api from '../api';
 import { logout } from '../account/accountActions';
 import { deadQueue } from '../session/sessionActions';
 import eventToAction from './eventToAction';
 import doEventActionSideEffects from './doEventActionSideEffects';
-import { tryGetActiveAccountState, tryGetAuth } from '../selectors';
+import { tryGetAuth } from '../selectors';
 import { BackoffMachine } from '../utils/async';
 import { ApiError } from '../api/apiErrors';
 import * as logging from '../utils/logging';
@@ -53,26 +52,32 @@ export const startEventPolling = (
 
   // eslint-disable-next-line no-constant-condition
   while (true) {
-    const globalState = assumeSecretlyGlobalState(getState());
-    const state = tryGetActiveAccountState(globalState);
-    const auth = state ? tryGetAuth(state) : undefined;
+    const auth = tryGetAuth(getState());
     if (!auth) {
-      // There is no logged-in active account.
+      // This account is not logged in.
       break;
     }
-    // `auth` represents the active account.  It might be different from
-    // the one in the previous loop iteration, if we did a backoff wait.
+    // Pre-#5005, `auth` secretly represents the active account.  It might
+    //   be different from the one in the previous loop iteration, if
+    //   we did a backoff wait.
     // TODO(#5009): Is that really quite OK?
+    // TODO(#5005): After #5005, it should be OK; remove these comments.
 
     let events = undefined;
     try {
       const response = await api.pollForEvents(auth, queueId, lastEventId);
       events = response.events;
 
       if (queueId !== getState().session.eventQueueId) {
-        // The user switched accounts or logged out.
-        // TODO(#5022): TODO(#5009): This doesn't seem like an adequate
-        //   check for that; it looks like it only detects a new REGISTER_COMPLETE.
+        // Pre-#5005, this will catch that the user has switched accounts,
+        //   as long as the old and new accounts' queue IDs don't collide.
+        //   It'll catch the switch on REGISTER_COMPLETE, which is a while
+        //   after the switch happens in the UI. But the essential thing
+        //   (again, pre-#5005) is that it's caught before we're set up with
+        //   a new loop for the new account, so that the old and new account
+        //   aren't writing to the same piece of state at the same time.
+        // TODO(#5005): Remove the "Pre-#5005" comment above.
+        // TODO(#5022): TODO(#5009): Catch a logout.
         break;
       }
     } catch (e) {