(branch 6.0.x and 5.0.x) Refactor SshProxyServer class to deal with https://github.com/apache/mina-sshd/pull/424. We have to change the logic of AbstractNioServiceFactory and its children and the way their instances are created in NioServiceWithPoolFactoryFactory, SharedNioServiceFactory and SharedNioServiceFactoryFactory methods.

This blocks integration of https://git.opendaylight.org/gerrit/c/netconf/+/110313 and its 5.0.x cherry-picks. The main branch does not contain concerned logic at all.