add experimental.arenas_create_ext mallctl

This mallctl accepts an arena_config_t structure which
can be used to customize the behavior of the arena.
Right now it contains extent_hooks and a new option,
metadata_use_hooks, which controls whether the extent
hooks are also used for metadata allocation.

The medata_use_hooks option has two main use cases:

1. In heterogeneous memory systems, to avoid metadata
being placed on potentially slower memory.

2. Avoiding virtual memory from being leaked as a result
of metadata allocation failure originating in an extent hook.

test/unit/edata_cache.c

@@ -5,7 +5,7 @@
 static void
 test_edata_cache_init(edata_cache_t *edata_cache) {
 	base_t *base = base_new(TSDN_NULL, /* ind */ 1,
-	    &ehooks_default_extent_hooks);
+	    &ehooks_default_extent_hooks, /* metadata_use_hooks */ true);
 	assert_ptr_not_null(base, "");
 	bool err = edata_cache_init(edata_cache, base);
 	assert_false(err, "");