Use UniqueLock in extract() instead of SharedLock. issue #280.

Author: greg7mdp

parallel_hashmap/phmap.h

@@ -3559,8 +3559,8 @@ class parallel_hash_set
         class K = key_type,
         typename std::enable_if<!std::is_same<K, iterator>::value, int>::type = 0>
     node_type extract(const key_arg<K>& key) {
-        SharedLock m;
-        auto it = this->template find<K, SharedLock>(key, this->hash(key), m);
+        UniqueLock m;
+        auto it = this->template find<K, UniqueLock>(key, this->hash(key), m);
         return it == end() ? node_type() : extract(const_iterator{it});
     }