fix(Backup): speed up backup listing

[fbeauchamp]

caching vm backup of a remote will launch the cache check and generation in parallel on all the backup of a rmote. If a remote have a huge number of backups, it may launch hundreds of generation in parallel, overloading the remote and xo-server process which will have to read, compress and write all the JSON at once.

To mitigate it we will only read 16 vm backup folder in parallel per remote.

Check list

Check if done, if not relevant leave unchecked.

• [ ] PR reference the relevant issue (e.g. Fixes #007 or See xoa-support#42)
• [ ] if UI changes, a screenshot has been added to the PR
• [ ] documentation updated
• CHANGELOG.unreleased.md:
  • [ ] enhancement/bug fix entry added
  • [ ] list of packages to release updated (${name} v${new version})
• I have tested added/updated features (and impacted code)
  • [ ] unit tests (e.g. cron/parse.spec.js)
  • [ ] if xo-server API changes, the corresponding test has been added to/updated on xo-server-test
  • [ ] at least manual testing

Process

1. create a PR as soon as possible
2. mark it as WiP: (Work in Progress) if not ready to be merged
3. when you want a review, add a reviewer (and only one)
4. if necessary, update your PR, and re- add a reviewer

From the Four Agreements:

1. Be impeccable with your word.
2. Don't take anything personally.
3. Don't make assumptions.
4. Always do your best.