When performing block import, we can batch state root verifications and
header checks, doing them only once per chunk of blocks, assuming that
the other blocks in the batch are valid by extension.
When we're not generating receipts, we can also skip per-transaction
state root computation pre-byzantium, which is what provides a ~20%
speedup in this PR, at least on those early blocks :)
We also stop storing transactions, receipts and uncles redundantly when
importing from era1 - there is no need to waste database storage on this
when we can load it from the era1 file (eventually).
242bbf03fc