Tweak to Radiation Optimization

* Optimized garbage collection of radiation sources until we get qdel() hints.
* Made the RADIATION_LOWER_LIMIT configurable (option for people to save some cpu time)
* Added some comments and broke the proccessor into separate procs for profiling purposes
* Added a changelog.

code/controllers/configuration.dm

@@ -720,6 +720,9 @@ var/list/gamemode_cache = list()
 					if(values.len > 0)
 						language_prefixes = values
 
+				if("radiation_lower_limit")
+					radiation_lower_limit = text2num(value)
+
 				else
 					log_misc("Unknown setting in configuration: '[name]'")