[bitshares/bitshares-core] Improve block generation performance (#1251)

Update: it’s even slower to replay after changed `transactions` in `signed_block` struct to `processed_transaction_with_signees` type. :-/ Code is here: https://github.com/abitmore/bitshares-core/commit/5b45f824de73846c5a8bc66442a738f87d291344

By the way, I tried to extract trx signees before pushing a block with this commit: https://github.com/abitmore/bitshares-core/commit/7309e8580a4d52764be1462a9369baf01de569b5, don’t know if it’s worthwhile. With the patch, I assume that the performance will be better when there is frequent chain reorganization, but perhaps will have reduced performance under normal conditions due to additional data copy, although both affect validation nodes only.