Milestone: mempool and block producer

This issue will be used to trace the dev process of mempool & block producer.

The goal is to run the very first testnet of opentron.🚀

## Update 2021/03/01

Since the producer is the most complex part, the milestone is divided into:

- [x] executor for all type of transactions
- [x] a state-db implementation: use `cargo run -- dev` to test against it
- [x] refactor channel protocol to handle transaction broadcast
- [x] impl `Manager::produceBlock` (in `manager` crate)
- [x] impl consensus base(code in `base/*.java`)
- most of the code logic is implemented by `manager` crate
- [ ] impl DPoS mining (logic in `dpos/*.java` files)
- [x] DposSlot - merge logic into `Manager`
- [x] IncentiveManager
- [x] MaintenanceManager
- [ ] StateManager
- [ ] DposService
- [x] DposTask - rewrite using `producer_service` crate

## Update 2021/03/15

- [ ] refactor channel protocol to handle incoming and outgoing broadcasts
- [ ] add a PeerContext to manage connected peers