+ to null.
+
+stomp_persistent: keep items across queue server restart, if enabled.
+
+softlimit: an absolute or relative "soft memory limit"; daemons will
+ restart themselves gracefully when they find they've hit
+ this amount of memory usage. Defaults to 90% of PHP's global
+ memory_limit setting.
+
+inboxes: delivery of messages to receiver's inboxes can be delayed to
+ queue time for best interactive performance on the sender.
+ This may however be annoyingly slow when using the DB queues,
+ so you can set this to false if it's causing trouble.
+
+breakout: for stomp, individual queues are by default grouped up for
+ best scalability. If some need to be run by separate daemons,
+ etc they can be manually adjusted here.
+
+ Default will share all queues for all sites within each group.
+ Specify as <group>/<queue> or <group>/<queue>/<site>,
+ using nickname identifier as site.
+
+ 'main/distrib' separate "distrib" queue covering all sites
+ 'xmpp/xmppout/mysite' separate "xmppout" queue covering just 'mysite'
+
+max_retries: for stomp, drop messages after N failed attempts to process.
+ Defaults to 10.
+
+dead_letter_dir: for stomp, optional directory to dump data on failed
+ queue processing events after discarding them.
+