IceStorm requires full participation on startup

[bentoi]

We have customers that shutdown-restart their system regularly.

If a machine hosting an IceStorm replica is un-available on restart, other IceStorm replicas won't start servicing requests because our replication mechanism require all the replicas to be up on full system startup (to ensure we use the latest DB).

In these use-cases, it can be acceptable if IceStorm doesn't use the latest DB and customers would prefer if they could restart the system even if not all the IceStorm instances are up.