pass through AsyncBufRead

[yoshuawuyts]

If AsyncBufRead is implemented on T, the stream should expose it. This makes some implementations much more efficient. Thanks!

[dignifiedquire]

To make this work we need to BufRead to native-tls::TlsStream

for that we need

• SecureTransport: start here: https://github.com/kornelski/rust-security-framework/blob/master/security-framework/src/secure_transport.rs#L1069
• openssl: start here: https://github.com/sfackler/rust-openssl/blob/master/openssl/src/ssl/mod.rs#L3547
• SChannel: already done: https://github.com/steffengy/schannel-rs/blob/master/src/tls_stream.rs#L918
• native-tls: start here: https://github.com/sfackler/rust-native-tls/blob/master/src/imp/security_framework.rs#L477

[yoshuawuyts]

Note that we may also need to forward Read for &T and BufRead for &T. Need to verify if we need this tho.

[link2xt]

tokio-rustls now passes AsyncBufRead through since version 0.26.2:
rustls/tokio-rustls@710cf25

Delta Chat currently can use both async-native-tls (when strict certificate checks are disabled) and tokio-rustls, but always wraps the stream with BufReader anyway because it may be async-native-tls.

Fixing this could prevent such double buffering.