FEATURE: Allow plugins to register demon processes (#11493)

This allows plugins to call `register_demon_process` with a Class inheriting from Demon::Base. The unicorn master process will take care of spawning, monitoring and restarting the process. This API should be used with extreme caution, but it is significantly cleaner than spawning processes/threads in an `after_initialize` block.

This commit also cleans up the demon spawning logging so that it uses the same format as unicorn worker logging. It also switches to the block form of `fork` to ensure that Demons exit after running, rather than returning execution to where the fork took place.

lib/demon/base.rb

@@ -141,15 +141,13 @@ class Demon::Base
   end
 
   def run
-    if @pid = fork
-      write_pid_file
-      return
+    @pid = fork do
+      Process.setproctitle("discourse #{self.class.prefix}")
+      monitor_parent
+      establish_app
+      after_fork
     end
-
-    Process.setproctitle("discourse #{self.class.prefix}")
-    monitor_parent
-    establish_app
-    after_fork
+    write_pid_file
   end
 
   def already_running?