Circumvent lack of 'close' event for tls sockets

It appears that in node.js (at least recent versions), the 'close' event
is not emitted by TLS sockets.  The CleartextStream class specifies that
the 'close' event is optional, so such operation is allowed.  In order
to compensate, the event can be trapped at the raw net.socket instead.

Fix mcavage/node-ldapjs#161