/Async/ismaster_ssl takes 5 minutes with OpenSSL

The addition of BIO_ssl_shutdown here:

https://github.com/mongodb/mongo-c-driver/commit/12120ec1cada

for CDRIVER-1147 causes some OpenSSL mock server tests to take 300 seconds to finish. The cause is mongoc_client_destroy requires the client to send an SSL shutdown message and receive the server's shutdown acknowledgment, but the mock server isn't able to detect the client shutdown.