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


Do you really want to remove `InMemoryHistoryNode`s from the underlying LinkedList? This might change the results if I then choose `statBasedSelectMetricHistory`. I believe both of these filters should not modify the underlying data but should merely return a filtered view (you could copy elements of interest into a new list, say).