CLEANUP: conn_stream: rename the cs_endpoint's context to "conn"

CLEANUP: conn_stream: rename the cs_endpoint's context to "conn"

This one is exclusively used by the connection, regardless its generic
name "ctx" is rather confusing. Let's make it a struct connection* and
call it "conn". This way there's no doubt about what it is and there's
no way it will be used by accident by being taken for something else.