Accelerate Hydra CI builds

[jpraynaud]

Why

The builds are very long in the Hydra CI and they often timeout. We need to accelerate the build times.

What

Accelerate build times in Hydra CI.

How

• Compile unit tests compiled in release in Hydra CI
• Remove unneeded crates builds. Keep only:
  • mithril-stm
  • mithril-common
  • mithril-signer
  • mithril-aggregator
  • mithril-client
  • mithril-client-cli
  • mithril-end-to-end