Use VirtualEventBase in mcrouter

Summary:
This switches mcrouter to use VirtualEventBase as the unified way of managing its threads. This allows multiple mcrouters to be run on the same thread-pool, without having to keep separate shutdown logic.

As an immediate win - this eliminates any special shutdown logic from tcc and standalone mcrouter.

Reviewed By: jmswen

Differential Revision: D4570982

fbshipit-source-id: 3974279005150e3e1db16230fde6b5951ae016b9

folly/fibers/EventBaseLoopController.h

@@ -114,6 +114,8 @@ class EventBaseLoopControllerT : public LoopController {
 };
 
 using EventBaseLoopController = EventBaseLoopControllerT<folly::EventBase>;
+using VirtualEventBaseLoopController =
+    EventBaseLoopControllerT<folly::VirtualEventBase>;
 }
 } // folly::fibers