unifying types: ngx_http_push_stream_worker_subscriber_t and ngx_http_push_stream_subscriber_t to reduce memory consumption

adding push_stream_longpolling_connection_ttl directive and make push_stream_ping_message_interval and push_stream_subscriber_connection_ttl directives available in location context

fix worker init and exit code to run only on process SINGLE or WORKER, to be possible use the module and proxy cache on the same nginx

adding function ngx_http_push_stream_run_cleanup_pool_handler and refactor on channel delete to avoid infinite loop

adding directive push_stream_footer_template to send a text to subscriber before close connection (channel delete or subscriber timeout)