Implement masque

[huitema]

Implementation of masque must be broken up in small pieces. These could include:

• parsing of URL templates, because they are used in the masque protocol
• possibly other infrastructure components, with unit tests
• basic UDP connect, with simple test showing end to end delivery of UDP datagrams -- maybe simple UDP echo program.
• QUIC over UDP connect, with a proxy server
• QUIC aware UDP connect
• UDP listen, tested with UDP echo
• QUIC over UDP listen
• QUIC aware UDP listen
• Multiplex QUIC aware UDP listen and QUIC aware UDP connect

The rule there is to always have a working build, making sure that nothing breaks.

[huitema]

First PR done: PR #1896, URL template.

[huitema]

The proxy intercept API is used to "hook" alternative processing of packets. It works at the low level of the protocol stack. The normal behavior is:

• the QUIC stack provides a timer indicating how long it needs to wait before packets can be sent.
• after that timer, the network stack calls the "prepare next packet" API.
• the QUIC stack attempts to prepare a packet, or a packet train
• if the packet can be prepared, the stack returns the packet length
• in all case, the stack resets the timer, indicating when to poll for the next packet.

The proxy intercept API is implemented within the "prepare next packet" API. It should only be called if the "next proxying time" value is lower than the current time. When called, it should:

• return the length of a formatted packet,
• set the "next proxying time".

The first task is to test that API:

• Create a connection, negotiate "Connect UDP".
• Use a text proxy -- maybe some variant of UDP echo protocol.
• Send a packet through the UDP connection when ready.
• Check that the packet is sent to the proxy.
• Retrieve the proxied packet on the proxy side.
• Submit a UDP echo on the selected port.
• let the proxy forward it.
• check that the proxy datagram receives it.

[huitema]

Lots of progress done, up to PR #1908. We can establish a QUIC connection through the proxy. Still to do:

• find a way to release unused UDP-CONNECT sessions, e.g., when the application is not using the path.
• verify that client properly obtain the "proxy IP and port" for a given mapping, in conformance with the RFC.
• maybe test with several connections through the proxy.

After that, the next step will probably be "QUIC aware", because it is the proper way of sharing a port. Also, it enables direct forwarding of "transformed" packets, which can unleash significant performance gains, and can also simplify PMTU discovery and ECN forwarding.