Revert "ngx_http_push_stream_timer_reset with a lock"

This reverts commit f8940a8d8970aafc460f4f45a0c2f3f6ecd07e20.
Revert "Calls to ngx_http_push_stream_send_old_messages should be surrounded by a lock"

This reverts commit c23100a293db025288ff2f29a4424a758320e6f1.