Store associated arena in tcache.

This fixes tcache_flush for manual tcaches, which wasn't able to find
the correct arena it associated with. Also changed the decay test to
cover this case (by using manually created arenas).

src/jemalloc.c

@@ -446,6 +446,7 @@ arena_migrate(tsd_t *tsd, unsigned oldind, unsigned newind) {
 	arena_nthreads_dec(oldarena, false);
 	arena_nthreads_inc(newarena, false);
 	tsd_arena_set(tsd, newarena);
+	tsd_iarena_set(tsd, newarena);
 }
 
 static void