Adding metric tracking framework for Azkaban

This change includes:-
1. Metric Tracking framework under Azkaban.metric package
2. Default metric reporting emitter under Azkaban.metric.inmemoryemitter
3. Exposed metric functionality /stats using statsservlet
4. Number of running flows and jobs as metrics

We will also have a curl option for following:-
* To set tracking interval for each metric
* To enable and disable metric tracking in Azkaban
* To set metric history details like maximum tracked interval and display points for /stats servlet


If there's nothing to initialize, just omit the entire method and rely on the default implementation in the GenericServlet superclass (StatsServlet -> HttpServlet -> GenericServlet).

(Also, if you did want to override the default implementation, it's good practice to annotate the method with `@Override`.)