ECIES

ECIES means encrypting with a public key and decrypting with a private key. This PR updates the network protocol of Copay to use ECIES rather than a shared secret. That way only the public key of the wallet author needs to be shared, and no password needs to be shared. This means even if somebody manages to get the public key (a passive adversary), they cannot decrypt the messages between the peers.

This is the first step of a multiple-step process of increasing the network security of Copay. The next step will be authentication.

Addresses issue https://github.com/bitpay/copay/issues/659