Git commit sha is displayed when --version flag is used and is written
to logs during service startup. Uses git_version crate when git is
available, and GIT_VERSION environment variable otherwise which is the case for docker
builds.
This change causes writer halves of a TLS stream to always flush after a
portion of bytes has been written by `std::io::copy`. Furthermore, some
cosmetic and minor functional changes are made to facilitate debug.
rsa_private_keys() function returns an empty vector when tries to read
pkcs8-encoded file instead of returning an error. So previous check was
failing on pkcs8. Leave only pkcs8 for now.
