Have network-specific critical flag for auto-updater

[tomusdrw]

Currently we have a single critical flag defined in util/version/Cargo.toml.
This is a pain if the update is critical on say kovan, but not on mainnet.

Instead we should store separate flags for each network and update https://github.com/paritytech/push-release to read those.

[tomusdrw]

I'd bump priority on this one.

[ascjones]

Should we aggregate network specific auto-updater settings e.g

[package.metadata.networks]
foundation = { forkBlock = 4370000, critical = false }
ropsten = { forkBlock = 10, critical = false }
kovan = { forkBlock = 6600000, critical = false }

or just add another section with critical flags for each network? Is the existing critical flag is only used by push-release?

[tomusdrw]

1. IMHO aggregate looks nicer.
2. Critical is only used by push-release