Bug#859930: apt-transport-https: Add http/2 support

[Tim Sattarov <stimur@gmail.com>]

Hello

I also think HTTP/2 could be useful in APT

> I don't think there is anything to gain from HTTP/2 support
> - we don't fetch small files, so all the multiplexing, binary
> framing, and header compression is basically irrelevant.

Here is my regular  "apt update" output:

~#  apt update
[skip]
Get:32 https://cloudfront.debian.net/debian experimental InRelease [107 kB]
Get:33 https://cloudfront.debian.net/debian stretch/contrib
Sources.diff/Index [27.8 kB]
Get:34 https://cloudfront.debian.net/debian stretch/main
Sources.diff/Index [27.9 kB]
Get:35 https://cloudfront.debian.net/debian stretch/non-free
Sources.diff/Index [27.8 kB]
Get:36 https://cloudfront.debian.net/debian stretch/main amd64
Packages.diff/Index [27.9 kB]
Get:37 https://cloudfront.debian.net/debian stretch/main i386
Packages.diff/Index [27.9 kB]
[skip ~170 lines of similar sized files]
Get:203 https://cloudfront.debian.net/debian unstable/contrib DEP-11
128x128 Icons [310
kB]                                                                                                             
 
Fetched 56.1 MB in 45s (1,225 kB/s)

I skipped about 170 lines of small, less than 50K files, they all could
be downloaded in parallel, instead of sequential order.
And I am downloading from HTTP/2 capable mirror (AWS CloudFront).

Unfortunately I'm not a developer, but let me know if I can help with
anything.

Rehards
Tim