Make authoring logic time independent

[bkchr]

Currently all authoring logic in Substrate assumes that we use milliseconds since the Unix epoch. We need to remove this requirement on milliseconds from the authoring logic of Aura/Babe. In case of a parachain we will want to use the relay chain block height as "time source". This is required, because we otherwise can not check the "time" when validating a block of a parachain on the relay chain.

I propose that we start with Aura as this is much simpler than Babe.

Implementation

1. Check the implementation for any usage of time.
2. Create some trait that can give you the current time. The trait should have the requirement that the time source is always strictly increasing.
3. Port Aura/Babe (while this can come later, depending on the complexity) to this new trait.

Required by

Aura for parachains: #115
Babe for parachains: #116