Revert "home: Don't use pointer to compute furthest_read_time."

This reverts commit 8f32db81a1.

This change unfortunately requires an index that we don't have, and
thus is incredibly expensive.  We'll need to do a thoughtful reworking
before we can integrate it again.

zerver/views/home.py

@@ -13,7 +13,7 @@ from zerver.forms import ToSForm
 from zerver.models import Message, Stream, UserProfile, \
     Realm, UserMessage, \
     PreregistrationUser, \
-    get_latest_read_usermessage
+    get_usermessage_by_message_id
 from zerver.lib.events import do_events_register
 from zerver.lib.actions import do_change_tos_version, \
     realm_user_count
@@ -223,7 +223,10 @@ def home_real(request: HttpRequest) -> HttpResponse:
             register_ret['pointer'] = register_ret['max_message_id']
         furthest_read_time = None
     else:
-        latest_read = get_latest_read_usermessage(user_profile)
+        latest_read = get_usermessage_by_message_id(user_profile, user_profile.pointer)
+        if latest_read is None:
+            # Don't completely fail if your saved pointer ID is invalid
+            logging.warning("User %s has invalid pointer %s" % (user_profile.id, user_profile.pointer))
         furthest_read_time = sent_time_in_epoch_seconds(latest_read)
 
     # We pick a language for the user as follows: