Silent JGroups transfer state protocol logs

Set level to error. As members can move on and leave
as soon as the expected instance count is reached, the
state transfer reference member can be unavailable,
which translates to an unnecessary warning.

References #282

Author: acogoluegnes

src/performance-tool/resources/logback.xml

@@ -7,6 +7,8 @@
 
     ${loggers}
 
+    <logger name="org.jgroups.protocols.pbcast.STATE_TRANSFER" level="error" />
+
     <root level="warn">
         <appender-ref ref="STDOUT" />
     </root>