log4j upgrade (sofastack#749)

pom.xml

@@ -67,7 +67,7 @@
         <jsr305.version>3.0.2</jsr305.version>
         <junit.dep.version>4.8.2</junit.dep.version>
         <junit.version>4.13.1</junit.version>
-        <log4j.version>2.15.0</log4j.version>
+        <log4j.version>2.17.0</log4j.version>
         <main.user.dir>${user.dir}</main.user.dir>
         <metrics.version>4.0.2</metrics.version>
         <mockito.version>1.9.5</mockito.version>