Change strand executor segment size to 64

Summary: This is an analogous change to D6665918 (https://github.com/facebook/folly/commit/4591832b4885f166236acee689dd2b11e3b3135b), which should reduce RSS variance due to extra allocated queue segments.

Reviewed By: yfeldblum

Differential Revision: D26770721

fbshipit-source-id: 7e83aaae1d5030c5ed50b34fae06236f0ead8ac1

folly/executors/StrandExecutor.h

@@ -116,7 +116,7 @@ class StrandContext : public std::enable_shared_from_this<StrandContext> {
       std::shared_ptr<StrandContext> thisPtr) noexcept;
 
   std::atomic<std::size_t> scheduled_{0};
-  UMPSCQueue<QueueItem, /*MayBlock=*/false> queue_;
+  UMPSCQueue<QueueItem, /*MayBlock=*/false, /*LgSegmentSize=*/6> queue_;
 };
 
 class StrandExecutor final : public SequencedExecutor {