Adds a shared flock guard used by all five cron entry points, keyed per

script and per install. Replaces the mail queue's own lock file, which
was not atomic and could be deleted out from under a long run. Bounds
the SMTP conversation so an unresponsive server cannot hold the lock.
Recovers rows left at Sending by a run that died, which nothing
previously picked up.

cron/ticket_email_parser.php

@@ -15,6 +15,10 @@ if (php_sapi_name() !== 'cli') {
     die("This script must be run from the command line.\n");
 }
 
+// Prevent overlapping runs of this script
+$cron_lock_script = __FILE__;
+require_once "../includes/cron_lock.php";
+
 // Autoload (Webklex & any composer deps)
 require_once "../libs/vendor/autoload.php";