migrate rename to folly::available_concurrency (assorted)

Summary:
X-link: facebookresearch/DCPerf#461

X-link: facebookincubator/nimble#484

The name `hardware_concurrency`, while parallel to `std::thread::hardware_concurrency`, may be misleading. Migrate to the name `available_concurrency`.

Reviewed By: Magoja

Differential Revision: D93262843

fbshipit-source-id: 5c8400fe8b4c8a4d7a4fb5f994a712ffdcae01e4

Author: yfeldblum

hphp/util/concurrent-lru-cache.h

@@ -241,7 +241,7 @@ template <class TKey, class TValue, class THash>
 ConcurrentLRUCache<TKey, TValue, THash>::
 ConcurrentLRUCache(size_t maxSize)
   : m_maxSize(maxSize), m_size(0),
-  m_map(folly::hardware_concurrency() * 4) // it will automatically grow
+  m_map(folly::available_concurrency() * 4) // it will automatically grow
 {
   m_head.m_prev = nullptr;
   m_head.m_next = &m_tail;

hphp/util/concurrent-scalable-cache.h

@@ -125,7 +125,7 @@ ConcurrentScalableCache(size_t maxSize, size_t numShards)
   : m_maxSize(maxSize), m_numShards(numShards)
 {
   if (m_numShards == 0) {
-    m_numShards = folly::hardware_concurrency();
+    m_numShards = folly::available_concurrency();
   }
   for (size_t i = 0; i < m_numShards; i++) {
     size_t s = maxSize / m_numShards;