Support disabling active requests counter
Support disabling active requests counter

Summary:
Disabling the active requests counter is known to significantly
improve Thrift performance. Add an option to loadgen ThriftServer to support
disabling the active requests tracker.

Differential Revision: D33517133

fbshipit-source-id: 39e70f5ed6f198d6e5056b75f0598b8a60a051f2

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift