pmix3x: add the PMIX_HASH_TABLE_FOREACH macro

this is a convenience macro similar to the PMIX_LIST_FOREACH macro,
that can be used to iterate on all the key/value pairs of a pmix_hash_table_t

(back-ported from upstream commit pmix/master@349971c68c)

opal/mca/pmix/pmix3x/pmix/src/class/pmix_hash_table.h

@@ -10,7 +10,7 @@
  * Copyright (c) 2004-2005 The Regents of the University of California.
  *                         All rights reserved.
  * Copyright (c) 2015-2016 Intel, Inc. All rights reserved.
- * Copyright (c) 2015      Research Organization for Information Science
+ * Copyright (c) 2015-2016 Research Organization for Information Science
  *                         and Technology (RIST). All rights reserved.
  * Copyright (c) 2016      Mellanox Technologies, Inc.
  *                         All rights reserved.
@@ -371,6 +371,29 @@ static inline int pmix_next_poweroftwo(int value)
 }
 
 
+/**
+ * Loop over a hash table.
+ *
+ * @param[in] key Key for each item
+ * @param[in] type Type of key (ui32|ui64|ptr)
+ * @param[in] value Storage for each item
+ * @param[in] ht Hash table to iterate over
+ *
+ * This macro provides a simple way to loop over the items in a pmix_hash_table_t. It
+ * is not safe to call pmix_hash_table_remove* from within the loop.
+ *
+ * Example Usage:
+ *
+ * uint64_t key;
+ * void * value;
+ * PMIX_HASH_TABLE_FOREACH(key, uint64, value, ht) {
+ *    do_something(key, value);
+ * }
+ */
+#define PMIX_HASH_TABLE_FOREACH(key, type, value, ht) \
+  for (void *_nptr=NULL;                                   \
+       PMIX_SUCCESS == pmix_hash_table_get_next_key_##type(ht, &key, (void **)&value, _nptr, &_nptr);)
+
 END_C_DECLS
 
 #endif  /* PMIX_HASH_TABLE_H */