removed push_stream_keepalive directive, let Nginx decide when to use keepalive and how many requests accept using keepalive_* directives

remove push_stream_shared_memory_cleanup_objects_ttl directive, using default value 10s, and 4s for cleanup timer

change push_stream_eventsource_support directive to be a subtype of push_stream_subscriber directive

change the message_queue and subscribers_queue type on the ngx_http_push_stream_worker_data_t structure