MINOR: conn-stream: Make cs_detach_* private and use cs_destroy() from outside

MINOR: conn-stream: Make cs_detach_* private and use cs_destroy() from outside

A conn-stream is never detached from an endpoint or an application alone,
except on a reset. Thus, to avoid any error, these functions are now
private. And cs_destroy() function is added to destroy a conn-stream. This
function is called when a stream is released, on the front and back
conn-streams, and when a health-check is finished.