Add special handling for URL-based variants of readValue() (ObjectMapper, ObjectReader)

[cowtowncoder]

(note: offshoot of #3455 -- read for background)

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 #3455 for details.

It seems like we might be able to handle some of those cases, specially readValue() variants of ObjectMapper / ObjectReader by using 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!)

[cowtowncoder]

Hmmh. Ok, this is not quite as easy to do as I thought. The problem being that the actual opening is within TokenStreamFactory (like JsonFactory) but that simply returns JsonParser instance.
Conversely ObjectMapper.readValue() etc do not have access to the connection being created etc.

But I think there is a way this could be achieved to some degree: if we implemented delegating InputStream that wraps underlying URL stream but overrides close() to use disconnect (in case where we have HttpURLConnection.
I will file an issue at jackson-core for this.

[cowtowncoder]

Replaced by:

FasterXML/jackson-core#803