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)

adding functions create_worker_subscriber_channel_sentinel_locked, create_channel_subscription and assing_subscription_to_channel_locked to group the logics responsable to add a subscriber to a channel

adding functions ngx_http_push_stream_send_response_message and ngx_http_push_stream_send_response_finalize to group the logics to send a message to subscriber and to finalize subscriber connection, respectively

put the code responsable for send old messages to subscriber on function ngx_http_push_stream_send_old_messages

changing ngx_http_push_stream_template_queue_t type and ngx_http_push_stream_find_or_add_template function to be possible to use ngx_http_push_stream_create_str on the copy of template

refactoring function ngx_http_push_stream_free_message_memory_locked and adding function ngx_http_push_stream_create_str to be easy create an ngx_str_t