Bug fix in SftpFileWriter and increased version constraints

[obemu]

• Bumped SDK constraints from ">=2.17.0 <3.0.0" up to ">=2.17.0 <4.0.0"

• Bug fix in SftpFileWriter for #50, #71, #100:

  When the input stream contains only one event and that event has fewer bytes than maxBytesOnTheWire, then the SftpFileWriter._doneCompleter never got completed. This happened because the completer got only completed in the _handleLocalData event handler if the _streamDone flag was already true, the problem is that if the stream only has one small event, then _handleLocalData will only be called once with the event and afterward _handleLocalDone will be called once, which sets _streamDone to true. At this point the _doneCompleter cannot be completed anymore because the _handleLocalData will not get called anymore. This problem causes never ending await statements when opening and writing a file.

  This issue has been fixed by checking if all sent bytes have already been acknowledged in the _handleLocalDone event handler and subsequently completing the _doneCompleter.

    void _handleLocalDone() {
      _streamDone = true;
      if (_bytesSent == _bytesAcked) {
        _doneCompleter.complete();
      }
    }

• Added return type SftpFileOpenMode to operator | in class SftpFileOpenMode. Fix for #80.

[vicajilau]

Thank you very much for your time in helping with this project. This has been integrated into a larger PR.