RequestContext: Declare gflag, add folly_ prefix to gflag name

Summary:
Declare gflag to enable turning it on and off at run time.
Added folly_ prefix to gflag name to reduce name pollution.

Reviewed By: davidtgoldblatt

Differential Revision: D19144459

fbshipit-source-id: 1e82cb102e47fea856952d1e630c03baf38adf1d

folly/io/async/Request.cpp

@@ -23,10 +23,9 @@
 
 #include <folly/MapUtil.h>
 #include <folly/SingletonThreadLocal.h>
-#include <folly/portability/GFlags.h>
 
 DEFINE_bool(
-    reqctx_use_hazptr,
+    folly_reqctx_use_hazptr,
     false,
     "RequestContext implementation using hazard pointers");
 
@@ -461,7 +460,7 @@ RequestContext::StateHazptr::Combined* RequestContext::StateHazptr::expand(
 }
 
 RequestContext::RequestContext()
-    : useHazptr_(FLAGS_reqctx_use_hazptr),
+    : useHazptr_(FLAGS_folly_reqctx_use_hazptr),
       rootId_(reinterpret_cast<intptr_t>(this)) {}
 
 RequestContext::RequestContext(const RequestContext& ctx, RootTag)

folly/io/async/Request.h

@@ -18,6 +18,7 @@
 
 #include <folly/Synchronized.h>
 #include <folly/container/F14Map.h>
+#include <folly/portability/GFlags.h>
 #include <folly/sorted_vector_types.h>
 #include <folly/synchronization/Hazptr.h>
 
@@ -26,6 +27,8 @@
 #include <mutex>
 #include <string>
 
+DECLARE_bool(folly_reqctx_use_hazptr);
+
 namespace folly {
 
 /*