requester: choose one sock able to respond to the wanted request type

[joelguittet]

Requester sends the wanted request type only to a suitable responder able to provide an answer, based on advertisement content.

See issue https://github.com/dashersw/cote/issues/240

Joel

[dashersw]

Thank you for the pull request @joelguittet!

respondsTo has historically been only used for the Sockend component, so this would be a breaking change. Cote has never had a breaking change before in 8 years, therefore this is not a light addition. One would need to deprecate the current use of respondsTo, and in a major version (v2) introduce this segregation. As I mentioned before, kotelett already has this dynamic message selection implemented, without the use of such a static field. I would very much love to continue this work there.