Several changes:
1) changed UserInfo return types for password and passphrase to byte[]s, to eliminate insecure Strings
2) added host key check config type "query", which bypasses the known_hosts file and instead always queries the UserInfo for confirmation (and added a UserInfo method to enable this query)
3) added an optional UserInfo method to return an Identity, to bypass the IdentityManager class (and make it possible to present a specific identity to the host)