Fix failure when median is enabled and --period is not set and logs are not sorted

Replacing duration_list with an itertool.chain object breaks the API
promise that it must have an append method. This cause all accumulate calls
to fail when simultaneously:
- median tracking is enabled
- period is not set and some scaling happens (ex: there is more than 4 days
  of data)
- log lines are being fed in a non-chronological order, where later lines
  are timestamped before the time some scaling happened (ex: log line for
  day 1, then day 5, then day 1 again)
When this happens, data accumulation will fail on these later lines,
causing the report to be only partial - for example errors will be missing.

apachedex/__init__.py

@@ -305,7 +305,7 @@ class APDEXStats:
         getattr(self, attribute) + getattr(other, attribute))
     self.duration_max = max(self.duration_max, other.duration_max)
     if self.enable_median:
-      self.duration_list = itertools.chain(self.duration_list, other.duration_list)
+      self.duration_list.extend(other.duration_list)
 
   def getApdex(self):
     if self.hit: