make last_message_time and last_message_tag global, so messages published in different channels on same second will receive sequential tags

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