Increase network i/o capacity (#5464)

The change increases the default network I/O worker thread pool size from 2 to 6. This will improve stability, as worker thread saturation correlates to desyncs, particularly on high-traffic peers, such as hubs.

src/xrpld/app/main/Application.cpp

@@ -256,8 +256,8 @@ public:
         if ((cores == 1) || ((config.NODE_SIZE == 0) && (cores == 2)))
             return 1;
 
-        // Otherwise, prefer two threads.
+        // Otherwise, prefer six threads.
-        return 2;
+        return 6;
 #endif
     }