Make export compression useful or get rid of it entirely

[dralley]

see: #4434

We could do any of the following:

• make the compression level configurable for users who really want it, even though it accomplishes little
• get rid of compression entirely, and therefore ditch the monkeypatches
• try out a more suitable algorithm such as zstd or lz4, which are much faster than e.g. gzip level 1 for similar amounts of compression

This would be a forwards-only change, no backports.

[mdellweg]

I'm fine with these suggestions. I only think we need to keep some versions being able to still read the current format, right?

[ipanova]

Not really, import/export is only compat between Z streams

[dralley]

As far as I'm aware, we only allow exports to be imported on the same Y stream they were produced on. So the only rule is that we can't remove compatibility with the gzipped export files from existing releases - but we can remove it in new releases, because those new releases just need to be able to import whatever format they're currently exporting.

[dralley]

It appears that Gzip at all (even using level 0 as we do) still incurs some overhead during imports, on the order of 12% or so in the test I did.

It's calculating a lot of CRC32 checksums internally, even though we are verifying integrity ourselves, and also it appears that it still needs to go through some "decompression" routines to seek around within the file.