update BABE slot time calculation on start-up

[noot]

Task summary

• substrate doesn't use relative time currently (ie. babe.Service.getCurrentSlot can be unused)
• when the node starts up, it should set the current slot as the slot based on non-relative time (unix time / slot duration)

[FlorianFranzen]

I would say it does not use relative time yet, as it is still the plan to implemented it. For now there are a few open questions around joining the network using relative time (e.g. w3f/polkadot-spec#263) and current development is mainly focused on parachains, but we will get back to it future.

Therefore I would recommend disabling it instead of throwing it all away.

[noot]

good to know, I'll update the issue description