Implement explicit tcache support.

Add the MALLOCX_TCACHE() and MALLOCX_TCACHE_NONE macros, which can be
used in conjunction with the *allocx() API.

Add the tcache.create, tcache.flush, and tcache.destroy mallctls.

This resolves #145.

include/jemalloc/internal/private_symbols.txt

@@ -425,6 +425,11 @@ tcache_get_hard
 tcache_maxclass
 tcache_salloc
 tcache_stats_merge
+tcaches
+tcaches_create
+tcaches_destroy
+tcaches_flush
+tcaches_get
 thread_allocated_cleanup
 thread_deallocated_cleanup
 tsd_booted