[SNMP4J] TSM transport models

[Stath, Paul]

I'm glad to see support for Transport Security Model (RFC5591) in SNMP4J 2.0.

It looks like communicating using TLS-TM (RFC6353) will work "out of the box" with SNMP4J 2.0.

However, it looks like communicating with a device supporting SSH-TM (RFC5592),
will require more work.

Firstly, it looks like only outgoing SSHTM connections can be creating using
DefaultSshTransportMapping(), since listen() is an empty method that doesn't
open any listen sockets, and isListening() always returns false.

Since there is no implementation of the SshTransportAdapter interface,
it is rather difficult to construct the DefaultSshTransportMapping() class with
the stock 2.0 SNMP4J library.  (grin)

Is it fair to say that the SSHTM implementation in SNMP4J is incomplete at this point, or
would it be expected to work if implementations of the SshSession<I> and SshTransportAdater<I>
interfaces were written to work with a 3rd party SSH java library like JSch or
Apache SSHD?