Consider using rustls vs. OpenSSL

[moderation]

I've been running sds as my Envoy control plane for a couple of months now and it has been great. One of the first things I did was switch to rustls vs. OpenSSL. I notice a large drop in binary size but more importantly sds used a lot less memory.

This tweet just reminded me about this - https://twitter.com/BRIAN_____/status/1146187144030896129

Rustls vs OpenSSL: (link: https://jbp.io/2019/07/01/rustls-vs-openssl-performance.html)

• 15% quicker to send data
• 5% quicker to receive data
• 20-40% quicker to full handshake (client)
• 10% quicker to full handshake (server)
• 30-70% quicker to resume (client)
• 10-20% quicker to resume (server)
• Uses <1/2 as much RAM

The only change required is in Cargo.toml

rusoto_dynamodb = { version = "0.40", default_features = false, features = ["rustls"] }

Happy to create a PR is this is of interest.