Request compressed rpm transfer

[bmwiedemann]

Request compressed rpm transfer, if server supports it. This can save 10-70% of transfer bandwidth, because (d)rpm metadata is uncompressed

E.g.

curl -L -v http://download.opensuse.org/update/leap/15.0/oss/noarch/kernel-source-vanilla-4.12.14-lp150.12.22.1_lp150.12.25.1.noarch.drpm | gzip -c1 | wc -c

compresses 11157036 bytes to 4161087

On the downside, it adds 32 byte to every HTTP request header
which would add 256 microseconds on a 1MBit/s line.

Also, none of the public openSUSE mirrors support compression of (d)rpm files yet and if they did, it could cause some server-side CPU load.

Note: Only tested downloading single rpm with zypper in -d http://aw.lsmod.de/rpm/test.rpm

Not yet tested with multicurl.

[mlschroe]

Please do this only for "small enough" rpms (i.e. there should be some threshold) and not for all files.

[mlschroe]

(we even know the header size, so we can do something clever)

[mlandres]

(we even know the header size, so we can do something clever)

The media backend does not know anything about the file (just the URL and maybe the expected size). And the .rpm from download.opensuse.org (via public openSUSE mirrors) are handled by multicurl (i.e. are not covered by the patch).