Use Relay Chain Blocks for Temporal Dependencies

[joepetrowski]

Periodic schedules (like promotion periods in the Fellowship) depend on numbers of blocks as their "clock", which are meant to correspond to some time periods like days, weeks, months, etc.

With both async backing and shared coretime coming, using the parachain block number will be unreliable.

We should switch all system parachains to use the Relay Chain block number for its HOURS, DAYS, etc. constants and migrate any existing state with values linked to the parachain block number.

[joepetrowski]

Linking to paritytech/polkadot-sdk#3268

[ggwpez]

For calculating time the relay block number will also not work, since there can be multiple para blocks per relay block 🙃
(responded to the other issue as well)

[xlc]

can we just use timestamp?