Scale to Zero

[cohix]

Constd should have a CONSTD_MIN_INSTANCES option which defaults to 1, but can be set to 0.

Setting this option to 0 would cause Constd to start up, but not launch any Sat instances.

In this mode, Constd should broadcast interests for all of the Runnables it knows about. When a message is received for a Runnable that isn't currently launched, it should hold the message in memory, launch the appropriate Sat instance, and then provide an endpoint for that new Sat to fetch the message on startup.

Sat should add an option SAT_COLDSTART_MESSAGE=true, which prompts it to reach out to Constd to fetch said message and handle it normally.