optimize server-side full handshakes for TLS 1.2 and 1.3

[bdaehlie]

We ran the Rustls benchmarking tool with the Rustls v0.23 release and Rustls appears to be a bit slower than OpenSSL 3.2.0 on server-side full handshakes for TLS 1.2 and 1.3. We should optimize until we are faster than OpenSSL.

[ctz]

Possible investigation ideas:

• add ECDSA or ED25519 support to the openssl-bench tool, and use that to see if it boils down to RSA signing performance, or something else.
• compare using rustls-libcrypto (a rustls CryptoProvider on top of OpenSSL libcrypto) to control for crypto library (nb. does not exist at the time of writing)