结束握手
https://tools.ietf.org/html/rfc6455#section-1.8
1.4. Closing Handshake
_This section is non-normative._
The closing handshake is far simpler than the opening handshake.
Either peer can send a control frame with data containing a specified
control sequence to begin the closing handshake (detailed in
Section 5.5.1). Upon receiving such a frame, the other peer sends a
Close frame in response, if it hasn't already sent one. Upon
receiving _that_ control frame, the first peer then closes the
connection, safe in the knowledge that no further data is
forthcoming.
After sending a control frame indicating the connection should be
closed, a peer does not send any further data; after receiving a
control frame indicating the connection should be closed, a peer
discards any further data received.
It is safe for both peers to initiate this handshake simultaneously.
The closing handshake is intended to complement the TCP closing
handshake (FIN/ACK), on the basis that the TCP closing handshake is
not always reliable end-to-end, especially in the presence of
intercepting proxies and other intermediaries.
By sending a Close frame and waiting for a Close frame in response,
certain cases are avoided where data may be unnecessarily lost. For
instance, on some platforms, if a socket is closed with data in the
receive queue, a RST packet is sent, which will then cause recv() to
fail for the party that received the RST, even if there was data
waiting to be read.