State machine, sync refactoring

State machine model:
- Functionality: transactions, transitions, coinbase, fees, state rebuild
- Tests
- Benchmarking
Mempool - basic fleshed out design
Sync
- Refactoring to build state sync
DAG
- Refactored single tip design into tracking two tips - headers (for light sync) and full tip (for state machine, mining, etc)
- Various helpers for state sync
Refactoring:
- Split blockdag.go into blockdag.go, blockdag_client.go, genesis.go
- Move things from types.go into block.go and tx.go

tinychainorg / tinychain