WIP: Http rework (#98)

- Move all HTTP tasks to a unified "MonitoredResource" model. This
   provides a uniform way of tracking object's lifetimes and disposing
   of them when complete.

 - Rewrite HTTP requests to use Netty instead of standard Java. This
   offers several advantages:
    - We have access to more HTTP verbs (mostly PATCH).
    - We can now do http -> https redirects.
    - We no longer need to spawn in a new thread for each HTTP request.
      While we do need to run some tasks off-thread in order to resolve
      IPs, it's generally a much shorter task, and so is less likely to
      inflate the thread pool.

 - Introduce several limits for the http API:
    - There's a limit on how many HTTP requests and websockets may exist
      at the same time. If the limit is reached, additional ones will be
      queued up until pending requests have finished.
    - HTTP requests may upload a maximum of 4Mib and download a maximum
      of 16Mib (configurable).

 - .getResponseCode now returns the status text, as well as the status
   code.

src/main/java/dan200/computercraft/shared/util/IoUtil.java

@@ -0,0 +1,24 @@
+/*
+ * This file is part of ComputerCraft - http://www.computercraft.info
+ * Copyright Daniel Ratcliffe, 2011-2019. Do not distribute without permission.
+ * Send enquiries to dratcliffe@gmail.com
+ */
+
+package dan200.computercraft.shared.util;
+
+import java.io.Closeable;
+import java.io.IOException;
+
+public class IoUtil
+{
+    public static void closeQuietly( Closeable closeable )
+    {
+        try
+        {
+            closeable.close();
+        }
+        catch( IOException ignored )
+        {
+        }
+    }
+}