Add "show caps stats by user" and "show caps stats by cap" console commands to print various counts of capability invocation by user and by cap

This currently prints caps requests received and handled, so that overload of received compared to handled or deadlock can be detected.
This involves making BaseStreamHandler and BaseOutputStream record the ints, which means inheritors should subclass ProcessRequest() instead of Handle()
However, existing inheriting classes overriding Handle() will still work, albeit without stats recording.
"show caps" becomes "show caps list" to disambiguate between show caps commands

OpenSim/Framework/Servers/HttpServer/BaseStreamHandler.cs

@@ -29,14 +29,35 @@ using System.IO;
 
 namespace OpenSim.Framework.Servers.HttpServer
 {
+    /// <summary>
+    /// Base streamed request handler.
+    /// </summary>
+    /// <remarks>
+    /// Inheriting classes should override ProcessRequest() rather than Handle()
+    /// </remarks>
     public abstract class BaseStreamHandler : BaseRequestHandler, IStreamedRequestHandler
     {
-        public abstract byte[] Handle(string path, Stream request,
-                                      IOSHttpRequest httpRequest, IOSHttpResponse httpResponse);
-
         protected BaseStreamHandler(string httpMethod, string path) : this(httpMethod, path, null, null) {}
 
         protected BaseStreamHandler(string httpMethod, string path, string name, string description)
             : base(httpMethod, path, name, description) {}
+
+        public virtual byte[] Handle(
+            string path, Stream request, IOSHttpRequest httpRequest, IOSHttpResponse httpResponse)
+        {
+            RequestsReceived++;
+
+            byte[] result = ProcessRequest(path, request, httpRequest, httpResponse);
+
+            RequestsHandled++;
+
+            return result;
+        }
+
+        protected virtual byte[] ProcessRequest(
+            string path, Stream request, IOSHttpRequest httpRequest, IOSHttpResponse httpResponse)
+        {
+            return null;
+        }
     }
 }