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


We can remove elements in  generalSelectMetricHistory, because generalSelectMetricHistory has a duplicated data. I have shortlisted data using time frame https://github.com/azkaban/azkaban/pull/376/files#diff-4c45b04406f9bb2e09d70380bff7f401R121