Add DebugAPI to Node, fix StateDB

- Allow starting the DebugAPI from the node via config
- In StateDB:
    - Make checkpoints when ProcessTxs() succeeds
    - Remove extra hardcoded `statedb` path that was redundant
    - Replace hardcoded `[:4]` by `[:]` when parsing idx, which failed because
      idx is 6 bytes length now.
- Extra: In node, use waitgroup instead of `stoppedXXX` channels to wait for
syncrhonizer goroutines to finish.

config/config.go

@@ -84,6 +84,9 @@ type Node struct {
 		ReceiptTimeout      Duration `validate:"required"`
 		IntervalReceiptLoop Duration `validate:"required"`
 	} `validate:"required"`
+	Debug struct {
+		APIAddress string
+	}
 }
 
 // Load loads a generic config.