RPC: add support for JSON-RPC 2.0-style request batching

If the top-level object is an array, it is assumed to be an array of
JSON-RPC requests.  An array is returned, containing one response (error or
not) per request, in the order submitted.

In a slight change in semantics, batched requests -always- return
an HTTP 200 OK status, even ones full of invalid or incorrect requests.

src/bitcoinrpc.cpp

@@ -2952,6 +2952,39 @@ void JSONRequest::parse(const Value& valRequest)
         throw JSONRPCError(-32600, "Params must be an array");
 }
 
+static Object JSONRPCExecOne(const Value& req)
+{
+    Object rpc_result;
+
+    JSONRequest jreq;
+    try {
+        jreq.parse(req);
+
+        Value result = tableRPC.execute(jreq.strMethod, jreq.params);
+        rpc_result = JSONRPCReplyObj(result, Value::null, jreq.id);
+    }
+    catch (Object& objError)
+    {
+        rpc_result = JSONRPCReplyObj(Value::null, objError, jreq.id);
+    }
+    catch (std::exception& e)
+    {
+        rpc_result = JSONRPCReplyObj(Value::null,
+                                     JSONRPCError(-32700, e.what()), jreq.id);
+    }
+
+    return rpc_result;
+}
+
+static string JSONRPCExecBatch(const Array& vReq)
+{
+    Array ret;
+    for (unsigned int reqIdx = 0; reqIdx < vReq.size(); reqIdx++)
+        ret.push_back(JSONRPCExecOne(vReq[reqIdx]));
+
+    return write_string(Value(ret), false) + "\n";
+}
+
 static CCriticalSection cs_THREAD_RPCHANDLER;
 
 void ThreadRPCServer3(void* parg)
@@ -3006,15 +3039,26 @@ void ThreadRPCServer3(void* parg)
         {
             // Parse request
             Value valRequest;
-            if (!read_string(strRequest, valRequest) || valRequest.type() != obj_type)
+            if (!read_string(strRequest, valRequest))
                 throw JSONRPCError(-32700, "Parse error");
 
-            jreq.parse(valRequest);
+            string strReply;
+
+            // singleton request
+            if (valRequest.type() == obj_type) {
+                jreq.parse(valRequest);
 
-            Value result = tableRPC.execute(jreq.strMethod, jreq.params);
+                Value result = tableRPC.execute(jreq.strMethod, jreq.params);
 
-            // Send reply
-            string strReply = JSONRPCReply(result, Value::null, jreq.id);
+                // Send reply
+                strReply = JSONRPCReply(result, Value::null, jreq.id);
+
+            // array of requests
+            } else if (valRequest.type() == array_type)
+                strReply = JSONRPCExecBatch(valRequest.get_array());
+            else
+                throw JSONRPCError(-32700, "Top-level object parse error");
+                
             conn->stream() << HTTPReply(200, strReply, fRun) << std::flush;
         }
         catch (Object& objError)