Switch to fine-grained reentrancy support.

Previously we had a general detection and support of reentrancy, at the cost of
having branches and inc / dec operations on fast paths.  To avoid taxing fast
paths, we move the reentrancy operations onto tsd slow state, and only modify
reentrancy level around external calls (that might trigger reentrancy).

src/arena.c

@@ -1966,11 +1966,9 @@ arena_new(tsdn_t *tsdn, unsigned ind, extent_hooks_t *extent_hooks) {
 		 * If we're here, then arena 0 already exists, so bootstrapping
 		 * is done enough that we should have tsd.
 		 */
-		int8_t *reentrancy_level = tsd_reentrancy_levelp_get(tsdn_tsd(
-		    tsdn));
-		++*reentrancy_level;
+		pre_reentrancy(tsdn_tsd(tsdn));
 		hooks_arena_new_hook();
-		--*reentrancy_level;
+		post_reentrancy(tsdn_tsd(tsdn));
 	}
 
 	return arena;