Provide a static page during bootup

[rgbkrk]

trafficstars

While the two ioloop.run_sync operations are running, provide a static page letting the user know the pool is warming up.

[minrk]

Two implementations come to mind:

1. start a temporary server with a single "Under construction, thanks for your patience" page (ideally with tiled construction GIFs) in a background thread while these complete, or
2. Don't use run_sync at all, put these in a threaded executor and short-circuit existing handlers until the execution has finished.

[rgbkrk]

ideally with tiled construction GIFs

You made my morning better.