This refactors the last bit of code that depends on the OpenSSL C
library. Note that the openssl-probe does not depend on OpenSSL, it only
tries to detect the CA certificate directory on the host system.
This still depends on unreleased diesel and diesel-async code. As soon
as new diesel and diesel-async code has been released, we can remove
the [patch.crates-io] from Cargo.toml.
