[downloader:http] add 'consume-content' option

* fix connection not being released when the response is neither
  successful nor retried
* add the ability to consume the HTTP response body instead of closing
  the connection

reference:

https://docs.python-requests.org/en/latest/user/advanced/#body-content-workflow

docs/configuration.rst

@@ -3616,6 +3616,25 @@ Description
     contains JPEG/JFIF data.
 
 
+downloader.http.consume-content
+---------------------------------
+Type
+    ``bool``
+Default
+    ``false``
+Description
+    Controls the behavior when an HTTP response is considered
+    unsuccessful
+
+    If the value is ``true``, consume the response body. This
+    avoids closing the connection and therefore improves connection
+    reuse.
+
+    If the value is ``false``, immediately close the connection
+    without reading the response. This can be useful if the server
+    is known to send large bodies for error responses.
+
+
 downloader.http.chunk-size
 --------------------------
 Type

gallery_dl/downloader/http.py

@@ -44,6 +44,12 @@ class HttpDownloader(DownloaderBase):
         self.mtime = self.config("mtime", True)
         self.rate = self.config("rate")
 
+        if not self.config("consume-content", False):
+            # this resets the underlying TCP connection, and therefore
+            # if the program makes another request to the same domain,
+            # a new connection (either TLS or plain TCP) must be made
+            self.release_conn = lambda resp: resp.close()
+
         if self.retries < 0:
             self.retries = float("inf")
         if self.minsize:
@@ -113,7 +119,7 @@ class HttpDownloader(DownloaderBase):
         while True:
             if tries:
                 if response:
-                    response.close()
+                    self.release_conn(response)
                     response = None
                 self.log.warning("%s (%s/%s)", msg, tries, self.retries+1)
                 if tries > self.retries:
@@ -170,6 +176,7 @@ class HttpDownloader(DownloaderBase):
                 if code in retry_codes or 500 <= code < 600:
                     continue
                 self.log.warning(msg)
+                self.release_conn(response)
                 return False
 
             # check for invalid responses
@@ -182,6 +189,7 @@ class HttpDownloader(DownloaderBase):
                     continue
                 if not result:
                     self.log.warning("Invalid response")
+                    self.release_conn(response)
                     return False
 
             # check file size
@@ -191,11 +199,13 @@ class HttpDownloader(DownloaderBase):
                     self.log.warning(
                         "File size smaller than allowed minimum (%s < %s)",
                         size, self.minsize)
+                    self.release_conn(response)
                     return False
                 if self.maxsize and size > self.maxsize:
                     self.log.warning(
                         "File size larger than allowed maximum (%s > %s)",
                         size, self.maxsize)
+                    self.release_conn(response)
                     return False
 
             build_path = False
@@ -284,6 +294,11 @@ class HttpDownloader(DownloaderBase):
 
         return True
 
+    def release_conn(self, response):
+        """Release connection back to pool by consuming response body"""
+        for _ in response.iter_content(self.chunk_size):
+            pass
+
     @staticmethod
     def receive(fp, content, bytes_total, bytes_start):
         write = fp.write