MINOR: conn-stream: Use a dedicated function to conditionally remove a CS

MINOR: conn-stream: Use a dedicated function to conditionally remove a CS

cs_free_cond() must now be used to remove a CS. cs_free() may be used on
error path to release a freshly allocated but unused CS. But in all other
cases cs_free_cond() must be used. This function takes care to release the
CS if it is possible (no app and detached from any endpoint).

In fact, this function is only used internally. From the outside,
cs_detach_* functions are used.