Some migrations should prevent startup

[brianolson]

Some migrations can run in the background but some migrations should prevent normal startup (no data fetching from algod, no data serving). Right now a backround migration followed by a stop-the-world migration can leave the system in an unworkable system while the backround migration runs. Do the dependency checking on all pending migrations and don't let the system start up if any pending migration is a stop-the-world migration.