[WORKER] Implement new STATS request

STATS requests will now return usage information for each LB
defined on a device (TCP, HTTP, or both). This change introduces
a new StatisticsManager class for managing temporary storage of
HAProxy statistics to deal with the fact that its reported values
are reset on restart. Also add tests for the new class.

Removed the LBStatistics class since it wasn't necessary.

Change-Id: I56177a829650b2206ee855fdf4756ed52825e936
Merge "[WORKER] Implement new STATS request"