Switch timestamp format to epoch millisecond (Integration with Luminol - part 1)

Step #1 for integrating with Luminol
* Update Cluster, GC, Jmeter, Netstat, Procmeminfo, Procvmstat, Proczoneinfo, Sar, Top and Metric to use epoch ms as the timestamp format.
* Update Client charting to use new epoch ms time format
* Implement CSV download functionality, which downloads csv data with date string timestamp instead of epoch ms. It also only downloads the data corresponding to the current time range in the chart
* Performance optimization for add chart

Please review/merge.
ts_format is the current format of timestamp. ts is the timestamp in epoch ms. So it looks ok. I will update the docstring