Fix: aggregator and signer blocked during Cardano transactions import

[jpraynaud]

Content

This PR includes a fix to the aggregator and signer nodes which are blocked during the database insertions of the Cardano transactions: the state machines and the REST API were completely inaccessible.

Pre-submit checklist

• Branch
  • [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

Issue(s)

Closes #1797

[github-actions[bot]]

Test Results

    4 files  ±0     51 suites  ±0   8m 34s :stopwatch: -10s 1 114 tests ±0  1 114 :white_check_mark: ±0  0 :zzz: ±0  0 :x: ±0  1 262 runs  ±0  1 262 :white_check_mark: ±0  0 :zzz: ±0  0 :x: ±0 

Results for commit 217f80c7. ± Comparison against base commit e67c8307.