http2 can keep upgraded connection up

CMakeLists.txt

lib/client.c

lib/handshake.c

lib/http2.c

lib/libwebsockets.c

lib/libwebsockets.h

lib/output.c

lib/private-libwebsockets.h

lib/server-handshake.c

lib/server.c

lib/service.c

test-server/test-server.c