Add batch metrics fetch support

### What?
This PR adds support for querying data for a list of metrics.

Documented the API and response format here:
https://github.com/rackerlabs/blueflood/wiki/10minuteguide#retrieve-data-for-list-of-metrics

### Changes
* Add BatchMetricsQueryHandler
 * Generic query handler that uses a thread pool and astyanax reader to make read queries.
 * Has ability to return partial results if the query timeout gets exceeded.
 * Added test cases

* Refactor existing HTTP parser and boiler plate for plot requests for reuse
 * Moved RollupsQueryParams to its own class 
 * Plot request validator moved into its own class

* Introduce BatchMetrics JSON output serializer 
 * Added test cases

* Added HTTP endpoint for batch fetch

* Added HTTP handler for batch metrics fetch
 * Added test cases

TODO
[] Add changelog to CHANGES file once PR is +1ed