cleanup fixes in linux-tcp

[pabigot]

There's a unit test for Monitor that could be included if #234 is merged first.

• Monitor: avoid null pointer dereference when copying instances

  When the copy constructor was added to allow passing a monitor by value
  into a lambda the implementation did not account for the possibility of
  the watchable having already been destroyed.

  Also provide the companion copy assignment to complete the triad.

• TcpResolver: reduce risk of accessing destructed TcpConnection

  Invoking TcpHandler::onError might result in the connection being
  destroyed. Though the reference to it in TcpClosed() is likely benign,
  it's safer to follow the standard practice of returning a nullptr to
  indicate that the connection is known to be destroyed.