Use temp files for Rollback

[aphralG]

Proposed changes

• Removed rollbackFileContents map[string][]byte which was used to rollback files during a failed config apply
• Instead write files that have beed updated or deleted to a temp Dir rollback<some-number> in /var/lib/nginx-agent/
• Clear all temp files and temp dirs after a config apply is complete in the ClearCache function

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING document
• I have run make install-tools and have attached any dependency changes to this pull request
• If applicable, I have added tests that prove my fix is effective or that my feature works
• If applicable, I have checked that any relevant tests pass after adding my changes
• If applicable, I have updated any relevant documentation (README.md)
• If applicable, I have tested my cross-platform changes on Ubuntu 22, Redhat 8, SUSE 15 and FreeBSD 13

[UnwashedMeme]

• we're destroying any file permissions it had when it gets moved. Read from sourcepath and apply to destpath?
• It's often better to create a temp file in the target location and then do a rename once the data is written. This way other processes don't see partially written files at expected locations.

[UnwashedMeme]

For a while now I've been wanting an NGINX with chroot option:

1. create a new chroot for NGINX
   • especially if you can take advantage of some underlying COW semantics, e.g. zfs, btrfs, overlayfs
2. test the new config nginx --chroot=/var/lib/nginx/chroots/$UUID -t /etc/nginx/nginx.conf
3. tell nginx to pivot and reload nginx --chroot=/var/lib/nginx/chroots/$UUID -- this can no longer be a simple SIGHUP.

But this would make these tasks so easy to do-- we can prepare the new space, and if it doesn't work there's no trying to copy files back into the right location.

I've been thinking maybe we could do this with a eBPF program that intercepts open calls to rewrite the path to the desired chroot and then step 3 becomes 1. replace the eBPF, 2. SIGHUP. Would need to ensure the newly forked workers receive the new eBPF and bonus points if the old workers maintain the chroot/eBPF-rewrite until they die.