This can happen when we call throwing version of
basic_stream_socket::remote_endpoint() call while client disconnected.

This option prevents the nghttp2 library from sending PING frame with
ACK flag set in the reply to incoming PING frame.  To allow the
application to send PING with ACK flag set, nghttp2_submit_ping() now
recognizes NGHTTP2_FLAG_PING in its flags parameter.

It is very hard to support multiple protocols in backend while
retaining multiple mode settings.  Therefore, we dropped modes except
for default and HTTP/2 proxy mode.  The other removed modes can be
emulated using combinations of options.  Now the backend connection is
not encrypted by default.  To enable encryption on backend connection,
use --backend-tls option.

nghttpx: Deprecate backend-http1-connections-per-frontend in favor of backend-connections-per-frontend

This is required to avoid session stall because of too slow frontend
connection.

We added 2 new option instead: --frontend-http2-max-concurrent-streams
and --backend-http2-max-concurrent-streams.