We should be able to support using OpenSSL to perform the actual encryption/decryption. See https://godoc.org/github.com/libp2p/go-openssl#Client and https://godoc.org/github.com/libp2p/go-openssl#Server.

Motivation: OpenSSL is faster and has better side-channel protection.

Requirement: This feature should only be enabled when the "openssl" build flag is enabled.