It fixes potential connection leak if some unsent data was left in the SSL
buffer.  Particularly, that could happen when a client canceled the stream
after the HEADERS frame has already been created.  In this case no other
frames might be produced and the HEADERS frame alone didn't flush the buffer.

Now it returns NGX_AGAIN if there's still data to be sent.

Checking for return value of c->send_chain() isn't sufficient since there
are data can be left in the SSL buffer.  Now the wew->ready flag is used
instead.

In particular, this fixed a connection leak in cases when all streams were
closed, but there's still some data to be sent in the SSL buffer and the
client forgot about the connection.

Particularly this fixes alerts on OS X and NetBSD systems when HTTP/2 is
configured over plain TCP sockets.

On these systems calling writev() with no data leads to EINVAL errors
being logged as "writev() failed (22: Invalid argument) while processing
HTTP/2 connection".

Previously, a stream could be closed by timeout if it was canceled
while its send window was exhausted.

It's useless to generate HEADERS if the stream has been canceled already.

Previously, if the worker process exited, GOAWAY was sent to connections in
idle state, but connections with active streams were closed without GOAWAY.

When the stream is terminated the HEADERS frame can still wait in the output
queue.  This frame can't be removed and must be sent to the client anyway,
since HTTP/2 uses stateful compression for headers.  So in order to postpone
closing and freeing memory of such stream the special close stream handler
is set to the write event.  After the HEADERS frame is sent the write event
is called and the stream will be finally closed.

Some events like receiving a RST_STREAM can trigger the read handler of such
stream in closing state and cause unexpected processing that can result in
another attempt to finalize the request.  To prevent it the read handler is
now set to ngx_http_empty_handler.

Thanks to Amazon.

There is no reason to add the "Content-Length: 0" header to a proxied request
without body if the header isn't presented in the original request.

Thanks to Amazon.

According to RFC 7540, an endpoint should not send more than one RST_STREAM
frame for any stream.

Also, now all the data frames will be skipped while termination.