Do a bit less work for pool_join.

core/thread/thread_pool.odin

@@ -103,14 +103,18 @@ pool_join :: proc(pool: ^Pool) {
 
 	yield()
 
+	// Because we already stopped the pool, there's no need to take a lock here.
+
 	started_count: int
 	for started_count < len(pool.threads) {
 		started_count = 0
 		for t in pool.threads {
 			if .Started in t.flags {
-				join(t)
 				started_count += 1
 			}
+			if .Joined not_in t.flags {
+				join(t)
+			}
 		}
 	}
 }