SuperVersion::Unref() to use sequential consistency to decrease ref counting

Summary: I'm not sure the expected results of std::atomic::fetch_sub() when using memory_order_relaxed, and I suspect TSAN complains.

Test Plan: make all check

Reviewers: rven, yhchiang, igor

Reviewed By: igor

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D32259

db/column_family.cc

@@ -181,7 +181,7 @@ SuperVersion* SuperVersion::Ref() {
 
 bool SuperVersion::Unref() {
   // fetch_sub returns the previous value of ref
-  uint32_t previous_refs = refs.fetch_sub(1, std::memory_order_relaxed);
+  uint32_t previous_refs = refs.fetch_sub(1);
   assert(previous_refs > 0);
   return previous_refs == 1;
 }