share extractor and downloader sessions

There was never any "good" reason for the strict separation
between extractors and downloaders. This change allows for
reduced resource usage (probably unnoticeable) and less lines
of code at the "cost" of tighter coupling.

gallery_dl/extractor/sankaku.py

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright 2014, 2015 Mike Fährmann
+# Copyright 2014-2017 Mike Fährmann
 #
 # This program is free software; you can redistribute it and/or modify
 # it under the terms of the GNU General Public License version 2 as
@@ -35,7 +35,6 @@ class SankakuTagExtractor(AsynchronousExtractor):
     def items(self):
         data = self.get_job_metadata()
         yield Message.Version, 1
-        yield Message.Headers, self.session.headers
         yield Message.Directory, data
         for image in self.get_images():
             image.update(data)