Add docs for thread-pool.* args, add flow to reject RTR clients

Add docs for thread-pool.* args, add flow to reject RTR clients

+An RTR client is rejected when there aren't available threads at the pool to attend it.
+Add new function at thread_pool.c to check if the pool has available threads to work.
+Use an internal buffer at sockaddr2str(), since the buffer received as parameter wasn't utilized by anybody.
+Update max values: thread-pool.server.max=500, thread-pool.validation.max=100.