In order to keep consistent with logback-appender and log4j2-appender, I modified pom.xml. Change log-loghub-producer's version from 0.1.4 to 0.1.8. Add protobuf 2.5.0 to pom.xml.
Rename parameter ioThreadsCount to maxIOThreadSizeInPool in order to keep consistent with log-loghub-producer.
Fixed flush issue through addShutdownHook.
Add a new class LoghubAppenderCallback, it will record the error message in org.apache.log4j.helpers.LogLog.