CLEANUP: conn-stream: Rename cs_conn_close() and cs_conn_drain_and_close()

These functions don't close the connection but only perform shutdown for
reads and writes at the mux level. It is a bit ambiguous. Thus,
cs_conn_close() is renamed cs_conn_shut() and cs_conn_drain_and_close() is
renamed cs_conn_drain_and_shut(). These both functions rely on
cs_conn_shutw() and cs_conn_shutr().
3 files changed