Fix aggregator stress test

[dlachaume]

Content

This PR includes a fix for the aggregator stress test.
Due to updates on the aggregator node, the bootstrap of the aggregator stress test was no longer working (e.g. here).
To succeed, an additional step was required, involving a restart of the aggregator and to move one epoch forward before the signing registration.

The results of the Github workflow that the aggregator stress test with 500 signers and 500 clients can be found here.

Pre-submit checklist

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

Issue(s)

Closes #1676

[github-actions]

Test Results

    3 files  ±0     43 suites  ±0   8m 34s ⏱️ -4s
  959 tests ±0    959 ✅ ±0  0 💤 ±0  0 ❌ ±0 
1 053 runs  ±0  1 053 ✅ ±0  0 💤 ±0  0 ❌ ±0 

Results for commit 4f3fc48. ± Comparison against base commit 24f749b.

♻️ This comment has been updated with latest results.

[jpraynaud]

LGTM 👍