Fix `prefixStats` global variable leaking

`prefixStats` is leaked through stats.js and was in global scope of event emitter without failing graphite backend by pure accident.
Now it is read from config.

backends/graphite.js

@@ -32,6 +32,7 @@ var prefixTimer;
 var prefixGauge;
 var prefixSet;
 var globalSuffix;
+var prefixStats;
 
 // set up namespaces
 var legacyNamespace  = true;
@@ -188,6 +189,7 @@ exports.init = function graphite_init(startup_time, config, events, logger) {
   prefixSet       = config.graphite.prefixSet;
   globalSuffix    = config.graphite.globalSuffix;
   legacyNamespace = config.graphite.legacyNamespace;
+  prefixStats     = config.prefixStats;
 
   // set defaults for prefixes & suffix
   globalPrefix  = globalPrefix !== undefined ? globalPrefix : "stats";
@@ -195,6 +197,7 @@ exports.init = function graphite_init(startup_time, config, events, logger) {
   prefixTimer   = prefixTimer !== undefined ? prefixTimer : "timers";
   prefixGauge   = prefixGauge !== undefined ? prefixGauge : "gauges";
   prefixSet     = prefixSet !== undefined ? prefixSet : "sets";
+  prefixStats   = prefixStats !== undefined ? prefixStats : "statsd";
   legacyNamespace = legacyNamespace !== undefined ? legacyNamespace : true;
 
   // In order to unconditionally add this string, it either needs to be

stats.js

@@ -151,7 +151,7 @@ config.configFile(process.argv[2], function (config, oldConfig) {
   l = new logger.Logger(config.log || {});
 
   // setup config for stats prefix
-  prefixStats = config.prefixStats;
+  var prefixStats = config.prefixStats;
   prefixStats = prefixStats !== undefined ? prefixStats : "statsd";
   //setup the names for the stats stored in counters{}
   bad_lines_seen   = prefixStats + ".bad_lines_seen";