[fragment,hls,f4m,dash,ism] improve fragment downloading

- resume immediately
- no need to concatenate segments and decrypt them on every resume
- no need to save temp files for segments

and for hls downloader:
- no need to download keys for segments that already downloaded

youtube_dl/downloader/http.py

@@ -20,10 +20,14 @@ from ..utils import (
 
 
 class HttpFD(FileDownloader):
-    def real_download(self, filename, info_dict):
+    def real_download(self, filename_or_stream, info_dict):
         url = info_dict['url']
-        tmpfilename = self.temp_name(filename)
+        filename = filename_or_stream
         stream = None
+        if hasattr(filename_or_stream, 'write'):
+            stream = filename_or_stream
+            filename = '-'
+        tmpfilename = self.temp_name(filename)
 
         # Do not include the Accept-Encoding header
         headers = {'Youtubedl-no-compression': 'True'}