hazptr: Increase thread cache capacity

Summary: Increase thread cache capacity from 3 to 6 hazard pointers to accommodate 2 ConcurrentHashMap iterators.

Reviewed By: yfeldblum

Differential Revision: D8595800

fbshipit-source-id: 53275c3cf6fb11c976a4da987d79ec33b9e5e617

folly/synchronization/HazptrThrLocal.h

@@ -69,7 +69,7 @@ class hazptr_tc_entry {
  */
 template <template <typename> class Atom>
 class hazptr_tc {
-  static constexpr uint8_t kCapacity = 3;
+  static constexpr uint8_t kCapacity = 6;
 
   hazptr_tc_entry<Atom> entry_[kCapacity];
   uint8_t count_{0};