Use short-lived/ephemeral connections instead of connection-less

[esune]

Investigate the possibility of using short-lived connections instead of the current connection-less approach to support the proof-request mechanism for VC-AuthN.

A short-lived connection is a "normal" connection that is established with a goal code specifying that it will be severed once the purpose it was created for is achieved. As an example, I could create a connection with the purpose of completing a proof-request, use that connection to submit the proof-request to the holder and delete that connection once done.

A couple of caveats to consider:

• success/failure scenarios need to be considered. In particular, in case of failure a problem report should be delivered on the connection before it is deleted.
• experience could be clunky if the holder does not know what to expect: connections remaining in their wallet beyond the initial purpose, problem reports not arriving (see above), maybe more

Relates to #203