MEDIUM: connection: make mux->detach() release the connection

MEDIUM: connection: make mux->detach() release the connection

For H2, only the mux's timeout or other conditions might cause a
release of the mux and the connection, no stream should be allowed
to kill such a shared connection. So a stream will only detach using
cs_destroy() which will call mux->detach() then free the cs.

For now it's only handled by mux_pt. The goal is that the data layer
never has to care about the connection, which will have to be released
depending on the mux's mood.