Use a daemon thread to flush logs. (#684)

CHANGES.md

@@ -8,6 +8,7 @@ Release Notes.
 * Remove `idleCount` tag in Alibaba Druid meter plugin.
 * Fix NPE in handleMethodException method of apm-jdk-threadpool-plugin.
 * Support for C3P0 connection pool tracing.
+* Use a daemon thread to flush logs.
 
 All issues and pull requests are [here](https://github.com/apache/skywalking/milestone/213?closed=1)
 

apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/logging/core/FileWriter.java

@@ -97,6 +97,7 @@ public class FileWriter implements IWriter {
             public void handle(Throwable t) {
             }
         }), "SkywalkingAgent-LogFileWriter");
+        logFlusherThread.setDaemon(true);
         logFlusherThread.start();
     }