HTTP/3 (HTTP over QUIC) support

[DemiMarie]

HTTP 3 has several compelling advantages over HTTP 1.1 and HTTP 2:

• HTTP3 runs over QUIC, which natively supports roaming. This is especially useful for mobile clients.
• HTTP3 supports native multiplexing, without the head-of-line blocking problem that HTTP2 suffers from.
• HTTP3 allows congestion control to be implemented in userspace, which allows new algorithms (such as BBR) to be adopted before OS kernels get support for them.

[PMaynard]

See #28 for discussion on HTTP/2. I've no idea, but I assume HTTP3 will require mechanisms found in HTTP2 as well.

[moritzbuhl]

FYI, I am working on a proof of concept for HTTP3 for some research project using linuxquic. I am happy once it supports GET requests and can send back a file.
https://github.com/moritzbuhl/httpd-portable/tree/quic