puppet: Increase minimum memory for multiprocess queue workers.

This should give some more room for systems that are still below 4GB
of RAM to use the lower-memory multithreaded mode, which is less
likely to have OOM kills (a very bad experience).

There should be little cost, as few systems are likely allocated with
memory in this range.

(cherry picked from commit a22f418827)

puppet/zulip/manifests/app_frontend_base.pp

@@ -131,7 +131,15 @@ class zulip::app_frontend_base {
   # This determines whether we run queue processors multithreaded or
   # multiprocess.  Multiprocess scales much better, but requires more
   # RAM; we just auto-detect based on available system RAM.
-  $queues_multiprocess_default = $zulip::common::total_memory_mb > 3500
+  #
+  # Because Zulip can run in the multiprocess mode with 4GB of memory,
+  # and it's a common instance size, we aim for that to be the cutoff
+  # for this higher-performance mode.
+  #
+  # We use a cutoff less than 4000 here to detect systems advertised
+  # as "4GB"; some may have as little as 4 x 1000^3 / 1024^2 ≈ 3815 MiB
+  # of memory.
+  $queues_multiprocess_default = $zulip::common::total_memory_mb > 3800
   $queues_multiprocess = zulipconf('application_server', 'queue_workers_multiprocess', $queues_multiprocess_default)
   $queues = [
     'deferred_work',