Optimize : request hostname to system only once and use DateFormatUtils for date formating

+ request hostname only once, when the JSONEventLayout class is initialized
+ use commons-lang DateFormatUtils that is thread safe (and commons-lang is already a dependency). Moreover, this part of the code is now consistent with what was made in the *logstash-logback-encoder*
+ bonus: organized imports (intellij idea style) + format code

+30% performance (average) compared to the "head", better performance too compared to the ThreadLocal version (https://github.com/logstash/log4j-jsonevent-layout/pull/9)