Bugfix: Sockets not properly reattached if reconnected during disconnect event.

Bugfix: Sockets not properly reattached if reconnected during disconnect event.

The problem was that Connection::on_close was calling Detach() directly
after executing the "disconnect" event. Since we had a boolean attach count,
this was leaving sockets detached even if they had reattached in during the
event.

 * Added many asserts in http.cc and net.cc to ensure that sockets are
   connected when they should be.

 * Changed ObjectWrap to use a reference count instead of boolean attached_
   value.

 * Fixed similar bug in Timer.