Refactor crypto to use OpenSSL for everything but Public-Key Cryptography

Based on Twitter, mailing list, our forum and other people, more than 2/3 of respondents thinks that people just need to use HSMs for Public-Key cryptography (e.g. storing private keys and operations that involve public and private keys on the HSM).

This is an issue that aims to simplify the cryptography to use OpenSSL for everything but Public-Key cryptography, where we will keep the choice. This will also make OpenSSL mandatory.