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


My bad I intended to take from before notification. I have moved `from` declaration before notifying manager. 

But yes, I also observed that we are not sure to complete the call in 2000 milliseconds. Actually, We need to use something like ConcJUnit, in order to completely solve race condition. But I found that we are using wait statements in all other tests so using a separate library for only one case would be an overkill ?