Memory leak in Cardano transactions signature/proof

[jpraynaud]

Why

There is a memory leak in the signature/proof generation of the Cardano transactions. A first investigation shows that it seems to be due to the Merkle tree used to sign that is not released from memory (~2GB leak on testing-preview)

What

Investigate and fix the memory leak

How

• [x] Deactivate the signature of Cardano transactions on testing-preview
• [ ] Investigate the memory leak
• [ ] Fix the memory leak
• [ ] Reactivate the signature of Cardano transactions on testing-preview