multi: force connections to get closed in close_all_connections

Several independent reports on infinite loops hanging in the
close_all_connections() function when closing a multi handle, can be
fixed by first marking the connection to get closed before calling
Curl_disconnect.

This is more fixing-the-symptom rather than the underlying problem
though.

Bug: https://curl.haxx.se/mail/lib-2016-10/0011.html
Bug: https://curl.haxx.se/mail/lib-2016-10/0059.html

Reported-by: Dan Fandrich, Valentin David, Miloš Ljumović