FTP cannot handle partial reads

Any attempt to use an FTP-backed file object for something that reads less than the entire file fails, with the following stacktrace:

Stacktrace:

Traceback (most recent call last):
  File "/home/qwertystop/.pyenv/versions/hmcc-invdb/lib/python3.7/site-packages/fsspec/implementations/ftp.py", line 251, in _fetch_range
    callback=callback,
  File "/home/qwertystop/.pyenv/versions/3.7.4/lib/python3.7/ftplib.py", line 447, in retrbinary
    callback(data)
  File "/home/qwertystop/.pyenv/versions/hmcc-invdb/lib/python3.7/site-packages/fsspec/implementations/ftp.py", line 239, in callback
    raise TransferDone
fsspec.implementations.ftp.TransferDone

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/home/qwertystop/.pyenv/versions/hmcc-invdb/lib/python3.7/site-packages/fsspec/implementations/ftp.py", line 256, in _fetch_range
    self.fs.ftp.abort()
  File "/home/qwertystop/.pyenv/versions/3.7.4/lib/python3.7/ftplib.py", line 267, in abort
    raise error_proto(resp)
ftplib.error_proto: 150 0.193 seconds (measured here), 1.29 Mbytes per second

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "sync_db.py", line 412, in <module>
    sync()
  File "sync_db.py", line 408, in sync
    send_to_db(files, tablename, colspec, conf["psql"], args.init, args.product)
  File "sync_db.py", line 299, in send_to_db
    header = next(reader)
  File "/home/qwertystop/.pyenv/versions/hmcc-invdb/lib/python3.7/site-packages/fsspec/spec.py", line 1449, in read
    out = self.cache._fetch(self.loc, self.loc + length)
  File "/home/qwertystop/.pyenv/versions/hmcc-invdb/lib/python3.7/site-packages/fsspec/caching.py", line 153, in _fetch
    self.cache = self.fetcher(start, end)  # new block replaces old
  File "/home/qwertystop/.pyenv/versions/hmcc-invdb/lib/python3.7/site-packages/fsspec/implementations/ftp.py", line 259, in _fetch_range
    self.fs.ftp._connect()
AttributeError: 'FTP' object has no attribute '_connect'

I initially suspected this to be a simple typo; changing self.fs.ftp._connect() to self.fs.connect() at line 259 of ftp.py is at least sufficient to prevent this specific error. However, at that point, the connection attempt reliably times out when I attempt to read an FTPFile through csv.reader, so I am unsure whether the issue is more complicated than that; I don't know enough of the FTP spec to judge that.

Start in fsspec/implementations/ftp.py, especially _fetch_range and its callback, then reproduce the partial-read path through fsspec/spec.py and csv.reader. Check how ftplib handles abort and reconnect after a ranged transfer. Done means an FTP-backed file can read less than the full file without the _connect error or a timeout, with regression coverage if the existing tests provide a suitable place.