Signed entity lock mechanism

[Alenar]

Content

This PR add a lock mechanism to make our state machines skip a signed entity when needed (ie: when we need to do a costly computation on a signed entity but we still want our state machines to run).

On aggregator the skip is done when it try to create open messages. On signer the skip is done when it check if it can sign a pending certificate.

Pre-submit checklist

• Branch
  • [x] Tests are provided (if possible)
  • [x] Crates versions are updated (if relevant)
  • [x] CHANGELOG file is updated (if relevant)
  • [x] Commit sequence broadly makes sense
  • [x] Key commits have useful messages
• PR
  • [x] No clippy warnings in the CI
  • [x] Self-reviewed the diff
  • [x] Useful pull request description
  • [x] Reviewer requested
• Documentation
  • [ ] Update README file (if relevant)
  • [ ] Update documentation website (if relevant)
  • [ ] Add dev blog post (if relevant)

Issue(s)

Closes #1693

[github-actions[bot]]

Test Results

    3 files  ± 0     44 suites  ±0   8m 50s :stopwatch: +16s 1 069 tests +11  1 069 :white_check_mark: +11  0 :zzz: ±0  0 :x: ±0  1 167 runs  +11  1 167 :white_check_mark: +11  0 :zzz: ±0  0 :x: ±0 

Results for commit 7fc0b215. ± Comparison against base commit 93c0a5e9.

:recycle: This comment has been updated with latest results.