store [nfc]: Re-query account after _registerQueueWithRetry async gap

I believe this is NFC because:

- the only fields we read are zulipVersion, zulipMergeBase, and
  zulipFeatureLevel
- we read those fields only to update them if needed
- nothing else could have written to those fields during the async
  gap; this register-queue codepath is the only codepath that writes
  to them, and we don't allow concurrent register-queues for an
  account

But we've been using an Account object from across an async gap, and
that looks like a code smell that calls for a fix.

Author: chrisbobbe

lib/model/store.dart

@@ -976,6 +976,8 @@ class UpdateMachine {
     final t = (stopwatch..stop()).elapsed;
     assert(debugLog("initial fetch time: ${t.inMilliseconds}ms"));
 
+    // `!` is OK because _registerQueueWithRetry would have thrown if no account
+    account = globalStore.getAccount(accountId)!;
     if (initialSnapshot.zulipVersion != account.zulipVersion
         || initialSnapshot.zulipMergeBase != account.zulipMergeBase
         || initialSnapshot.zulipFeatureLevel != account.zulipFeatureLevel) {