Refactor/fix arenas manipulation.

Abstract arenas access to use arena_get() (or a0get() where appropriate)
rather than directly reading e.g. arenas[ind].  Prior to the addition of
the arenas.extend mallctl, the worst possible outcome of directly
accessing arenas was a stale read, but arenas.extend may allocate and
assign a new array to arenas.

Add a tsd-based arenas_cache, which amortizes arenas reads.  This
introduces some subtle bootstrapping issues, with tsd_boot() now being
split into tsd_boot[01]() to support tsd wrapper allocation
bootstrapping, as well as an arenas_cache_bypass tsd variable which
dynamically terminates allocation of arenas_cache itself.

Promote a0malloc(), a0calloc(), and a0free() to be generally useful for
internal allocation, and use them in several places (more may be
appropriate).

Abstract arena->nthreads management and fix a missing decrement during
thread destruction (recent tsd refactoring left arenas_cleanup()
unused).

Change arena_choose() to propagate OOM, and handle OOM in all callers.
This is important for providing consistent allocation behavior when the
MALLOCX_ARENA() flag is being used.  Prior to this fix, it was possible
for an OOM to result in allocation silently allocating from a different
arena than the one specified.

src/huge.c

@@ -50,7 +50,11 @@ huge_palloc(tsd_t *tsd, arena_t *arena, size_t usize, size_t alignment,
 	 * it is possible to make correct junk/zero fill decisions below.
 	 */
 	is_zeroed = zero;
-	arena = choose_arena(tsd, arena);
+	arena = arena_choose(tsd, arena);
+	if (unlikely(arena == NULL)) {
+		base_node_dalloc(node);
+		return (NULL);
+	}
 	ret = arena_chunk_alloc_huge(arena, NULL, csize, alignment, &is_zeroed);
 	if (ret == NULL) {
 		base_node_dalloc(node);