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/senmanga.py

@@ -1,6 +1,6 @@
 # -*- coding: utf-8 -*-
 
-# Copyright 2016 Mike Fährmann
+# Copyright 2016-2017 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
@@ -38,7 +38,6 @@ class SenmangaChapterExtractor(Extractor):
         data = self.get_job_metadata()
         yield Message.Version, 1
         yield Message.Directory, data
-        yield Message.Headers, self.session.headers
         for i in range(int(data["count"])):
             page = str(i+1)
             data["page"] = page