MINOR: connection: add cs_close() to close a conn_stream

MINOR: connection: add cs_close() to close a conn_stream

This basically calls cs_shutw() followed by cs_shutr(). Both of them
are called in the most conservative mode so that any previous call is
still respected. The CS flags are cleared so that it can be reused
(this is important for connection retries when conn and CS are reused
without being reallocated).