Support for more modern crypto schemes

[creshal]

ECDSA/Curve25519 for KEX, SHA2 as MAC and AES-GCM/ChaCha20 as cipher are getting more widespread, and I'd rather not drag along the old baggage on my servers for backwards compatibility with FireFTP (like we did with SSL3…).

I know this is far from a small request, but it'll be necessary sooner or later. The reduced CPU load from ed25519 would be a nice side effect, as FireFTP is cripplingly slow right now.

[mimecuvalo]

Thanks for the feedback. Indeed, I'd like to add this as well. I'm very busy these days, so in the meantime patches are happily accepted! A good place to start is: https://github.com/paramiko/paramiko/