[plugins] speedup journal collection (v2)

[champtar]

Instead of generating all the logs and tailing the last 100M, we get the first 100M of 'journalctl --reverse' that we then reverse again using our own implementation of tac.

To handle multiline logs we would need to use "tac -brs '^[^ ]'" that takes ~30s on 100M of logs when plain 'tac' takes ~0.3s. Our simple implementation in python takes 0.7s, and avoid an extra dependency.

On journalctl timeout we now get the most recents logs.

During collection logs are now buffered on disk, so we use 2xsizelimit. While running our tac we could actually truncate the source file to limit disk usage. Previously buffering was in RAM (also 2xsizelimit).

On my test server, logs plugin runtime goes from 34s to 9.5s.

Fixes #3615

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[packit-as-a-service]

Congratulations! One of the builds has completed. 🍾

You can install the built RPMs by following these steps:

• sudo yum install -y dnf-plugins-core on RHEL 8
• sudo dnf install -y dnf-plugins-core on Fedora
• dnf copr enable packit/sosreport-sos-3879
• And now you can install the packages.

Please note that the RPMs should be used only in a testing environment.