fix redis check memory leak (fixes #325)

Since tags is a reference to a list, and we're appending tags to the list,
we keep adding the same tags over and over again, and the aggregator thinks
they're separate contexts, so it keeps creating new aggregation buffers for
them. Fix is to dedupe tags in the redis check

checks.d/redisdb.py

@@ -108,8 +108,9 @@ def _get_conn(self, host, port, password):
 
     def _check_db(self, host, port, password, custom_tags=None):
         conn = self._get_conn(host, port, password)
-        tags = custom_tags or []
-        tags += ["redis_host:%s" % host, "redis_port:%s" % port]
+        tags = set(custom_tags or [])
+        tags = sorted(tags.union(["redis_host:%s" % host,
+                                  "redis_port:%s" % port]))
 
         # Ping the database for info, and track the latency.
         start = time.time()