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/ctl.c

@@ -1342,7 +1342,7 @@ thread_arena_ctl(tsd_t *tsd, const size_t *mib, size_t miblen, void *oldp,
 			tcache_t *tcache = tsd_tcache_get(tsd);
 			if (tcache != NULL) {
 				tcache_arena_reassociate(tsd_tsdn(tsd), tcache,
-				    oldarena, newarena);
+				    newarena);
 			}
 		}
 	}