allow tuning *IdleConn* for intra-cluster messages (#8799)

* allow tuning *IdleConn* for inter cluster messages

* default MaxIdleConnsPerHost to 128

config/default.json

@@ -323,7 +323,10 @@
         "UseExperimentalGossip": false,
         "ReadOnlyConfig": true,
         "GossipPort": 8074,
-        "StreamingPort": 8075
+        "StreamingPort": 8075,
+        "MaxIdleConns": 100,
+        "MaxIdleConnsPerHost": 128,
+        "IdleConnTimeoutMilliseconds": 90000
     },
     "MetricsSettings": {
         "Enable": false,