Make mempool fast

Same as #68 but actionable. It turns out that mempool is major sourse of slowdown at high TX rate. To alleviate that we need:

- [ ] Filter mempool asynchronously. Synchrounous filtering creates unnecessary delays in consensus
- [ ] Discard transaction from commited block immediately instead of verifying them
- [x] Use latest state of BCh for filtering. Right now we update state too late. Also this should avoid duplication of work
- [ ] Add multiple stages of TX verification. For example we need fast way to rule out obviously invalid TX. We shouldn't recheck properties that don't depend on blockchain state (signature is valid for example)