Feature request: connection namespaces

[Sorunome]

While it is good that connections are being re-used based on the (address, port) identifier, in some cases you might want to have a dart program create multiple separate sessions to the same remote (address, port). For that, some kind of namespace could be introduced to overgo that, of course being hidden by default

[shamblett]

Yes, sounds like a good idea.

[Sorunome]

I actually thought if implementing this, if you are ok with this. would stick it into the DefaultCoapConfig then

[shamblett]

Yes fine by me, can't think of anything better. Maybe pass it on construction to make it a bit more flexible but then again that's still as ort of configuration I guess.

[Sorunome]

done in #40