Re #74: Created StringBuilders thread local holder and using thread local StringBuilder in Log4jAdapterLabelPrinter that can be shared between many threads.

log4j2-appender/src/main/java/pl/tkowalcz/tjahzi/log4j2/labels/Log4jAdapterLabelPrinter.java

@@ -20,7 +20,7 @@ public class Log4jAdapterLabelPrinter implements LabelPrinter {
 
     @Override
     public void append(LogEvent event, Consumer<CharSequence> appendable) {
-        outputBuffer.setLength(0);
+        StringBuilder outputBuffer = StringBuilders.threadLocal();
 
         for (int i = 0; i < formatters.size(); i++) {
             formatters.get(i).format(event, outputBuffer);