Use relaxed order for EventBase::loopThread_

Summary:
[Folly] Use `relaxed` order for `EventBase::loopThread_` consistently. There is only one location which uses a different memory order.

Atomic operations on this location do not guard any other locations, so non-`relaxed` memory orders are not required.

Reviewed By: andriigrynenko

Differential Revision: D18722614

fbshipit-source-id: b73b61e4845c34c58e9f4adbeb368c815b375d38

folly/io/async/EventBase.cpp

@@ -306,7 +306,7 @@ bool EventBase::loopBody(int flags, bool ignoreKeepAlive) {
   std::chrono::microseconds busy;
   std::chrono::microseconds idle;
 
-  loopThread_.store(std::this_thread::get_id(), std::memory_order_release);
+  loopThread_.store(std::this_thread::get_id(), std::memory_order_relaxed);
 
   if (!name_.empty()) {
     setThreadName(name_);