Now using SFTP stream wrapper from phpseclib

- Upgraded phpseclib to master version (post 0.3.5)
- Now using fopen() on sftp URL for both read and write
- Fixes #4063
Removed unused writeBack method
Merge pull request #5954 from owncloud/extstorage-stream-sftp

[Ext storage] Now using SFTP stream wrapper from phpseclib