queue: Restore prematurely-cut upgrade path.

Revert c8f034e9a "queue: Remove missedmessage_email_senders code."
As the comment in the code says, it ensures a smooth upgrade path
from 1.7.x; we can delete it in master after 1.8.0 is released.
The removal commit was merged early due to a communication failure.

puppet/zulip/manifests/base.pp

@@ -46,6 +46,7 @@ class zulip::base {
              'error_reports',
              'feedback_messages',
              'invites',
+             'missedmessage_email_senders',
              'email_senders',
              'missedmessage_emails',
              'missedmessage_mobile_notifications',

puppet/zulip_ops/files/nagios3/conf.d/services.cfg

@@ -555,6 +555,15 @@ define service {
         contact_groups                  admins
 }
 
+define service {
+        use                             generic-service
+        service_description             Check missedmessage_email_senders queue processor
+        check_command                   check_remote_arg_string!manage.py process_queue --queue_name=missedmessage_email_senders!1:1!1:1
+        max_check_attempts              3
+        hostgroup_name                  frontends
+        contact_groups                  admins
+}
+
 define service {
         use                             generic-service
         service_description             Check email_senders queue processor

scripts/nagios/check-rabbitmq-consumers

@@ -50,6 +50,7 @@ queues = {
     'invites',
     'message_sender',
     'missedmessage_emails',
+    'missedmessage_email_senders',
     'email_senders',
     'missedmessage_mobile_notifications',
     'outgoing_webhooks',

tools/test-queue-worker-reload

@@ -17,7 +17,7 @@ from typing import IO, Text
 # TODO: Convert this to use scripts/lib/queue_workers.py
 TOOLS_DIR = os.path.dirname(os.path.abspath(__file__))
 successful_worker_launches = [
-    '[process_queue] 19 queue worker threads were launched',
+    '[process_queue] 20 queue worker threads were launched',
 ]
 
 def check_worker_launch(logfile_name):

zerver/worker/queue_processors.py

@@ -308,6 +308,19 @@ class EmailSendingWorker(QueueProcessingWorker):
             del copied_event['failed_tries']
         send_email_from_dict(copied_event)
 
+@assign_queue('missedmessage_email_senders')
+class MissedMessageSendingWorker(EmailSendingWorker):
+    """
+    Note: Class decorators are not inherited.
+
+    The `missedmessage_email_senders` queue was used up through 1.7.1, so we
+    keep consuming from it in case we've just upgraded from an old version.
+    After the 1.8 release, we can delete it and tell admins to upgrade to 1.8
+    first.
+    """
+    # TODO: zulip-1.8: Delete code related to missedmessage_email_senders queue.
+    pass
+
 @assign_queue('missedmessage_mobile_notifications')
 class PushNotificationsWorker(QueueProcessingWorker):
     def consume(self, data):