CDRIVER-1409 openssl_check_closed was always true

BIO_get_shutdown doesn't do what I thought: it doesn't check whether the
socket has been closed, it returns the state of a config flag, which
was always true. Caused /Client/exhaust_cursor/single tests to fail
with "error: connection closed".

It was only this test that failed because the driver only calls
check_closed in single-client mode on a socket idle for 1 second, and
only since #3c81f4f6 does this test pause a full second.