[API][ADMIN_API] Make VIP delete retry

Sometimes VIP delete fails and we are left with stale VIPs in the DB.
This patch retries when this event happens instead of giving up straight away.

Change-Id: I2c55abc96cc52335f162a6ab45dcafcf7b8e90d9
[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