SSLClient::~SSLClient() freed the TLS context before shutting down
the SSL session. mbedTLS sessions hold a raw pointer into the
context's mbedtls_ssl_config, so a live keep-alive session's
close_notify would read freed memory. Shut down the session first,
then free the context.
Add a regression test that destructs an SSLClient while a keep-alive
mbedTLS session is still open.
873d701972
