Revert "Implement concurent write requests."

This reverts commit 9acff6202cf130015308a741ed11ec05bbd206d1.
Get rid of Requester.doRequest(), replace with request() method that returns the response promise. Make getTimeout() part of the interface.
In SFTPEngine / Requester, move from using TimeUnit.SECONDS to TimeUnit.MILLISECONDS, and start using some more explicit naming
Add 'unconfirmed writes' feature to SFTP RemoteFileOutputStream, allowing for major speedups

Thanks to @romainreuillon for the idea and initial implementation! #87