Mock relay block number in dev service.

[JoshOrndorff]

Currently the dev service always injects the relay chain block number of 0. This is fine for many purposes, but the more our para logic depends on the relay block number the less suitable this becomes.

We now have at least three concrete cases that could benefit from a more realistically-mocked block number.

1. Crowdloan rewards are vested based on relay block number.
2. Runtime upgrades are allowed (or not) based on relay block number
3. Nimbus slots are triggered based on relay block number.

Checklist

• Does it require a purge of the network?
• You bumped the runtime version if there are breaking changes in the runtime ?
• Does it require changes in documentation/tutorials ?

[4meta5]

Looks good

[JoshOrndorff]

It would be good to have an integration test for this too. From typescript we should ensure that para block 1 has relay block 1000 and that parablock 2 has relay block 1002.

[joelamouche]

@JoshOrndorff Sounds good but that would mean using polkadot-launch right?

[JoshOrndorff]

This PR is all about the dev service, so no need to launch a parachain.