Lower log level of frequently-executed flow stats code

Change-Id: I2901a8ede86531677d506c54f7ee2878ef62bb49
diff --git a/core/net/src/main/java/org/onlab/onos/net/flow/impl/FlowRuleManager.java b/core/net/src/main/java/org/onlab/onos/net/flow/impl/FlowRuleManager.java
index 803abb9..95bf8c5 100644
--- a/core/net/src/main/java/org/onlab/onos/net/flow/impl/FlowRuleManager.java
+++ b/core/net/src/main/java/org/onlab/onos/net/flow/impl/FlowRuleManager.java
@@ -305,7 +305,7 @@
                 if (event == null) {
                     log.debug("No flow store event generated.");
                 } else {
-                    log.debug("Flow {} {}", flowEntry, event.type());
+                    log.trace("Flow {} {}", flowEntry, event.type());
                     post(event);
                 }
             } else {