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)

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