Database connections could wait for availability instead of failing directly

[fmartingr]

Databases could use a Wait() where they try to check for database readyness before conecting, instead of just connecting directly (and failing if not ready).

• This should be configurable behind a flag (--wait-for-db that defaults to false)
• While doing so, figure out a way to propagate the database state to other shiori domains, so we can create a proper liveness endpoint that returns a non-ok status code if the database is not ready.

From #587