Fix some weirdness in `offchain_worker`

We call `offchain_worker` with the state of the imported block and pass
the header of this block. However in the runtime we call all
`offchain_worker` functions with the number of the parent block. Besides
that we also pass all digests and not only the pre runtime digests. In
the context where the offchain worker is executed we have all digests, so
there is no real reason to only pass pre runtime digests. Another fix is
that we also insert the hash of the current header into the block hash map.