Do not let cache manager requests kill SMP Squid using isOpen() assertion.

Do not let cache manager requests kill SMP Squid using isOpen() assertion.

As the comment above the close call implies, we have not imported the foreign
socket descriptor into our fd_table yet. We must use raw close(2), just like
the corresponding Mgr::Request::Request(msg) code that allocates request.conn,
uses raw assignment to give that half-baked connection a descriptor.

TODO: This direct manipulation of Connection::fd is ugly, and this half-baked
connection will most likely cause more [hidden] problems down the road. For
example, Mgr::Request destructor will assert in a similar way if the request
object is destroyed before Action::respond() is called.