Improve cache upload speed on high-speed networks

Release notes

• The related merge request that addresses this issue disables transport-layer compression in gitlab-runner cache_client.go

Summary

Well, perhaps not a strict bug, but definitely a major performance degradation that seems unnecessary :-)

I have been trying to diagnose slow cache operations, and after hacking the gitlab-runner-helper binaries with more timing information I found that the download part of the cache_extractor operations can be extremely slow. This appears to be do to a combination of small buffers and the Golang built-in version of http not being optimized for bandwidth.

Steps to reproduce

• Add timing operations around the io.Copy() operation in routine download() of commands/helpers/cache_extractor.go.
• Use a local S3 server, without SSL (to enable highest-possible performance).
• When downloading a 931MB cache file with the 'mc' client outside of gitlab-runner, it takes 1.9seconds (10Gb network), so network and S3 server setup is A-OK.

Actual behavior

• Gitlab-runner takes 42 seconds to download the same file.

Expected behavior

• When I hack the code to use the Minio Golang package instead of presigned URLs + HTTP, the download takes 1.9 seconds in gitlab-runner too.

Environment description

• Using our own runners in a k8s cluster
• Ceph-backed radosgw S3 storage in the same cluster
• No SSL to improve bandwidth

Used GitLab Runner version

13.1.0 (development)

Possible fixes

If there's interest, we could probably provide a MR to use Minio instead, although there could be some extra complications to handle GCS natively too.