Shrink size of HashSkipList buckets from 56B to 48B (#13424)

Summary:
Previous order of fields in SkipList:

`const uint16_t kMaxHeight_;  // 2B`
`const uint16_t kBranching_;  // 2B`
`const uint32_t kScaledInverseBranching_;  // 4B`
`Comparator const compare_;  // 8B`
`Allocator* const allocator_;  // 8B`
`Node* const head_;  // 8B`
`std::atomic<int> max_height_;  // 4B`
`// 4B padding added automatically for alignment`
`Node** prev_;  // 8B`
`int32_t prev_height_;  // 4B`
`// 4B padding added automatically for alignment`

= 56B in total. By swapping prev_ and prev_height_, we get the following:

`const uint16_t kMaxHeight_;  // 2B`
`const uint16_t kBranching_;  // 2B`
`const uint32_t kScaledInverseBranching_;  // 4B`
`Comparator const compare_;  // 8B`
`Allocator* const allocator_;  // 8B`
`Node* const head_;  // 8B`
`std::atomic<int> max_height_;  // 4B`
`int32_t prev_height_;  // 4B`
`Node** prev_;  // 8B`

= 48B in total. So this change saves 8B per SkipList object. When allocated using AllocateAligned (as is the case for the [hash skiplist](https://github.com/facebook/rocksdb/blob/main/memtable/hash_skiplist_rep.cc#L243)) and assuming alignof(std::max_align_t) = 16, this change saves an additional 8B per SkipList object (so 16B in total).

Note: this does not affect the "skiplist" memtable, which internally uses InlineSkipList

Pull Request resolved: #13424

Reviewed By: cbi42

Differential Revision: D70423252

Pulled By: pdillinger

fbshipit-source-id: 450dcc7f0e9e86cd3481f6930e83eea5fef78b97

memtable/skiplist.h

@@ -134,8 +134,8 @@ class SkipList {
   // i up to max_height_ is the predecessor of prev_[0] and prev_height_
   // is the height of prev_[0].  prev_[0] can only be equal to head before
   // insertion, in which case max_height_ and prev_height_ are 1.
-  Node** prev_;
   int32_t prev_height_;
+  Node** prev_;
 
   inline int GetMaxHeight() const {
     return max_height_.load(std::memory_order_relaxed);