The graceful shutdown is implemented through a new 'closing'
state. This state is entered under different circumstances
depending on the protocol.
The gun:shutdown/1 function is now implemented and documented.
It allows shutting down the connection gracefully regardless
of the current state of the connection and for all protocols.
The behavior is entirely dependent on the protocol.
For HTTP/1.1 the connection stays up only until after the
current stream is complete; other streams are immediately
canceled.
For HTTP/2 a GOAWAY frame is sent and existing streams
continue to be processed. The connection is closed after
all streams are processed and the server's GOAWAY frame
is received.
For Websocket a close frame is sent. The connection is
closed when receiving the server's close frame.
In all cases the closing_timeout option defines how long
we wait, as a maximum, before closing the connection after
the graceful shutdown was started.
The graceful shutdown is also initiated when the owner
process goes away; when sending an HTTP/1.1 request
with the connection: close header; when receiving an
HTTP/1.1 response with the connection: close header;
when receiving an HTTP/1.0 response without a connection
header; when the server sends a GOAWAY HTTP/2 frame;
or when we send or receive a Websocket close frame.
Along with these changes, the gun:ws_send/2 function
now accepts a list of frames as argument. Those frames
may include a close frame that initiates the graceful
shutdown.