Narrow mcrouter library namespace imports

Summary: Replace broad namespace directives in two mcrouter library headers with the single types each header needs: `TType` in Carbon field metadata and `RPCTracer` in the Artillery handler API.

Reviewed By: stuclar

Differential Revision: D115077253

fbshipit-source-id: 421504cc5e7665317123986d24486be4078d0174

Author: Dan Melnic

mcrouter/lib/carbon/Fields.h

@@ -129,7 +129,7 @@ struct IsKVContainer {
   static constexpr bool value = IsOfTraitFieldType<T, FieldType::Map>::value;
 };
 
-using namespace apache::thrift::protocol;
+using apache::thrift::protocol::TType;
 
 constexpr TType CarbonToThriftFields[14] = {
     TType::T_STOP,