Compressed apt index files by default for 18.04?

[Julian Andres Klode]

Hi,

I'd like us to try out using LZ4 compressed index files in
/var/lib/apt/lists for 18.04, starting from the beginning
of the development period with apt 1.6~alpha1 (ASAP). 

This is done by swapping the default for Acquire::gzipIndexes
from false to true.

On my (Debian) system, this compresses /var/lib/apt/lists
from 1.3 GB down to 241 MB, which is a lot of space.

There are some packages broken with this, I just started
a mass bug filing in Debian for that purpose. I don't have
any equivalent to codesearch.d.n for Ubuntu, so I can't search
there, but I hope there are not more packages here (especially
since compressed indexes comes from Ubuntu in the first place).
The Debian bugs are here:

https://bugs.debian.org/cgi-bin/pkgreport.cgi?users=deity@lists.debian.org;tag=apt-internals

Some packages might have performance regressions: For example,
iterating over packages in python-apt and then accessing their
records causes random access to package files, which needs to
be emulated by seeking to the beginning and starting decompression
again. I'm sure we can fix this, however.

If we notice too many issues with that, we can turn it off again
before the release - this does not automatically decompress all
the files again, though, so devel users still need to manually do
that if they have issues.

Opinions?

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