Related to: https://github.com/spring-cloud/stream-applications/issues/171
It might not be necessary in real world, but with `SshServer` we fail
when we disconnect the session and cannot connect it back because
that server decides to die for a strange reason like no connections.
* Upgrade to SSHD 2.7.0
* Wrap default SFTP SF into a `CachingSessionFactory` to keep sessions opened
and don't let the server to die because on connections from clients
* Reenable SFTP Supplier tests
e3775e4c1a