BUG/MINOR: checks: attempt clean shutw for SSL check

BUG/MINOR: checks: attempt clean shutw for SSL check

Strict interpretation of TLS can cause SSL sessions to be thrown away
when the socket is shutdown without sending a "close notify", resulting
in each check to go through the complete handshake, eating more CPU on
the servers.

[wt: strictly speaking there's no guarantee that the close notify will
 be delivered, it's only best effort, but that may be enough to ensure
 that once at least one is received, next checks will be cheaper. This
 should be backported to 1.7 and possibly 1.6]