Correct the Exim queue threshold for Mailman

In change I9e21eb19b85c9a4b19f5e6c5d03a89c89028bbc4 we incorrectly
adjusted Exim's smtp_accept_max_per_host option from 10 to 50 rather
than setting the smtp_accept_queue_per_connection option we actually
needed. Add this parameter as a new tunable in the role, set the
other option back to its prior value, and update comment references
accordingly.

Change-Id: I13f0275202eba8b5190a76bff921f1ac5adbeea0

inventory/service/group_vars/mailman3.yaml

@@ -25,8 +25,9 @@ borg_backup_excludes_extra:
 exim_queue_interval: '1m'
 exim_queue_run_max: '50'
 exim_smtp_accept_max: '100'
+exim_smtp_accept_max_per_host: '10'
 # This should be set to more than mailman's mta.max_recipients value.
-exim_smtp_accept_max_per_host: '50'
+exim_smtp_accept_queue_per_connection: '50'
 exim_routers:
   - mailman_verp_router: |
       {% raw -%}

playbooks/roles/base/exim/README.rst

@@ -52,3 +52,11 @@ Installs and configures the exim mail server
    code 421. The option’s default value imposes no limit. If this option is
    set greater than zero, it is required that
    :zuul:rolevar:`exim.exim_smtp_accept_max` be non-zero.
+
+.. zuul:rolevar:: exim_smtp_accept_queue_per_connection
+   :default: null
+
+   The maximum number of delivery processes Exim will start for incoming SMTP
+   calls before it shunts them to periodic queue processing instead. If
+   the value is set to zero, no limit is applied. If this option is not set,
+   Exim will implicitly apply a default value of 10.

playbooks/roles/base/exim/templates/exim4.conf.j2

@@ -334,6 +334,11 @@ smtp_accept_max = {{ exim_smtp_accept_max }}
 smtp_accept_max_per_host = {{ exim_smtp_accept_max_per_host }}
 {% endif %}
 
+{% if exim_smtp_accept_queue_per_connection is defined %}
+# threshold for immediate delivery processing
+smtp_accept_queue_per_connection = {{ exim_smtp_accept_queue_per_connection }}
+{% endif %}
+
 ######################################################################
 #                       ACL CONFIGURATION                            #
 #         Specifies access control lists for incoming SMTP mail      #

playbooks/roles/mailman3/templates/mailman-extra.cfg.j2

@@ -5,7 +5,7 @@
 # a human.
 site_owner: "root@{{ ansible_fqdn }}"
 
-# We set this to less than the Exim's smtp_accept_max_per_host value.
+# We set this to less than the Exim's smtp_accept_queue_per_connection value.
 [mta]
 max_recipients: 45