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.

include/jemalloc/internal/tcache.h

@@ -109,6 +109,7 @@ void	tcache_bin_flush_small(tcache_bin_t *tbin, index_t binind, unsigned rem,
 void	tcache_bin_flush_large(tcache_bin_t *tbin, index_t binind, unsigned rem,
     tcache_t *tcache);
 void	tcache_arena_associate(tcache_t *tcache, arena_t *arena);
+void	tcache_arena_reassociate(tcache_t *tcache, arena_t *arena);
 void	tcache_arena_dissociate(tcache_t *tcache);
 tcache_t *tcache_get_hard(tsd_t *tsd);
 tcache_t *tcache_create(arena_t *arena);