Member recovery shares encryption with RSA-OAEP-256

[jumaffre]

Resolves #1720

Member recovery shares are now encrypted in the public:ccf.shares table with RSA, using (as opposed to NaCl's crypto box).

This has the following implications:

• The network's encryption key is no longer required (as we only encrypt with the target's - i.e. member's - public key).
• The submit_recovery_share.sh script has been updated and step-cli is no longer required (openssl does RSA-OAEP-256 key unwrapping fine).
• The 25519 and crypto box specific code has been removed.
• New RSAKeyPair class that can be used for unwrapping and corresponding RSAPublicKey for unwrapping (see tls/rsa_key_pair.h).

Also:

• Renamed keyshare in the public:ccf.members table to encryption_pub_key, as the previous name was confusing.

Next:

• Update js_generic.cpp to use new tls::RSAPublicKey to wrap keys.

[jumaffre]

This has been tested manually with AKV and the steps to wrap/unwrap a key are available here: #1720 (comment). The unwrapping operation is performed by the submit_recovery_share.sh script using openssl.

[ccf-bot]

rsa_encryption@14965 aka 20201030.15 vs master ewma over 50 builds from 14436 to 14954

[achamayou]

API schema needs to be updated