Improve handling of URL-backed InputStreams, closing

[cowtowncoder]

(note: replaces FasterXML/jackson-databind#3520)

It looks like JDK's HttpURLConnection has unfortunate handling of some error cases, leading to leakage of not-fully-closed HTTP connections under some error conditions. See FasterXML/jackson-databind#3455 for details.

It seems like we might be able to handle some of those cases if we could do something like:

URLConnection connection = new URL(url).openConnection();
try (InputStream stream = connection.getInputStream()) {
  // parse JSON
} finally {
  // java.net.HttpURLConnection.HttpURLConnection
  if (connection instanceof HttpURLConnection) {
    ((HttpURLConnection) connection).disconnect();
  }
}

(as suggested by @fxha -- thanks!)

But due to separation of concerns, this cannot (or should not) be done in databind. Instead we could wrap URL-backed InputStream in these cases, delegated all calls except for close(): and in close() we would do disconnect.