* Make it possible to calculate the storage as often as you want

So, until now each Substrate based blockchain has calculated the storage
root once, at the end of the block. Now there is Frontier that wants to
calculate some intermediate storage root. However this failed on block
import. The problem with that was the extrinsics root. When building the
block we stored `Default::default()` as extrinsics root, because yeah,
we don't know the extrinsics root before finishing the block. At the end
this extrinsics root was then calculated. But on block import we passed
the already known extrinsics root. This was no problem, as we removed
this value at the end of the block. However when you all the storage
root in between, that changes the storage root between block building
and block import.

This pr changes this behavior. It removes the `ExtrinsicsRoot` storage
entry and also doesn't pass it anymore to `System::initialize`. By doing
it, we remove the difference in the storage and fix the storage root mismatch.

* Fix bug with incorrectly calculating the extrinscs root

* Review feedback