Simplify SshdFtpChannel

Apache MINA sshd has simpler API for reading directories, and it has a
functional interface suitable for us. So no need to use our own
interface, or to deal with low-level abstractions like CloseableHandle.

Change-Id: Ic125c587535670504983f157a696b41ed6a76bb7
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>