[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/common.py

@@ -327,21 +327,22 @@ class FileDownloader(object):
             os.path.exists(encodeFilename(filename))
         )
 
-        continuedl_and_exists = (
-            self.params.get('continuedl', True) and
-            os.path.isfile(encodeFilename(filename)) and
-            not self.params.get('nopart', False)
-        )
+        if not hasattr(filename, 'write'):
+            continuedl_and_exists = (
+                self.params.get('continuedl', True) and
+                os.path.isfile(encodeFilename(filename)) and
+                not self.params.get('nopart', False)
+            )
 
-        # Check file already present
-        if filename != '-' and (nooverwrites_and_exists or continuedl_and_exists):
-            self.report_file_already_downloaded(filename)
-            self._hook_progress({
-                'filename': filename,
-                'status': 'finished',
-                'total_bytes': os.path.getsize(encodeFilename(filename)),
-            })
-            return True
+            # Check file already present
+            if filename != '-' and (nooverwrites_and_exists or continuedl_and_exists):
+                self.report_file_already_downloaded(filename)
+                self._hook_progress({
+                    'filename': filename,
+                    'status': 'finished',
+                    'total_bytes': os.path.getsize(encodeFilename(filename)),
+                })
+                return True
 
         min_sleep_interval = self.params.get('sleep_interval')
         if min_sleep_interval: