fix: reply with sni-specific certs

[rosahaj]

Currently the proxy uses the CA certificate for all TLS connections with the client, instead of generating a specific certificate for each host.

Example:

./japroxy &
curl --proxy localhost:8080 --cacert cert.pem https://duckduckgo.com/

# Output
curl: (60) SSL: certificate subject name 'localhost' does not match target host name 'duckduckgo.com'

This PR ensures a new certificate matching the host is generated on each request. On proxy startup, a temporary SessionKey is generated to sign all certificates (which should be faster than generating a new key for each certificate). cloudflare/cfssl is used for certificate generation and signing and elliptic curve private keys are used wherever possible.

[LyleMi]

Thanks for your pull request! Your contribution is greatly appreciated and has been a valuable addition to our project.