perf(storages): an ISO uploads to an SSH compute without waiting for each write

Each 32 MiB chunk went out as 1024 SFTP writes, each waiting for its
answer: 19 MiB/s on a gigabit LAN, ~1.9 s per chunk with the SSH session.
The writes now go out without waiting and their answers are read as they
come and at the end: ~0.53 s per chunk, the same file (checked by sha256).

paramiko's own pipelined writes (set_pipelined) drop the errors in their
answers, so a full disk would leave a short ISO behind a successful
upload. Each write is registered to its file instead, the way paramiko
does for prefetched reads: every answer, in any order, reaches the file
and an error is raised as soon as one is read.

storages/tests.py

@@ -523,6 +523,25 @@ class StorageUploadSecurityTests(TestCase):
             def read(self):
                 return bytes(self.buf)
 
+            # the SFTP client calls of a pipelined write: the file is its own client
+            MAX_REQUEST_SIZE = 32768
+            handle = b"handle"
+            sock = MagicMock(recv_ready=lambda: False)
+
+            @property
+            def sftp(self):
+                return self
+
+            def _async_request(self, fileobj, kind, handle, offset, data):
+                self.seek(offset)
+                self.write(data)
+
+            def _finish_responses(self, fileobj):
+                pass
+
+            def _check_exception(self):
+                pass
+
             def seek(self, pos):
                 self.pos = pos