INT-3412 (S)FTP Append, rmdir, Client Access

JIRA: https://jira.spring.io/browse/INT-3412

Initial commit - review only.

TODO:
- SFTP Tests
- Namespace/Adapter support for file append
- Docs
Well, looks like it should be reworked to the:
```
@ContextConfiguration
@RunWith(SpringJUnit4ClassRunner.class)
@DirtiesContext
```
For better performance