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

> IMO at this time 4% replay time hurts more than block generation performance.

Yes, **at this time** the chain is not under any meaningful pressure.

In the long run, faster block generation means higher throughput, aka higher TPS. This PR is the first step towards multi-threaded signature verification. If we’re going to do another stress testing, I think we need something like this.