Move `create_inherents` into the block-builder

This moves the `create_inherents` call into the block-builder. This has
the advantage that `create_inherents` will be able to reuse the same
context that will be used when applying the extrinsics and we also save
one call to `on_initialize`. To make sure that `create_inherents` does
not modify any state, we execute it in a transaction that is
rolled-back after doing the runtime call.