Update Synchronizer (node) implementation

- node:
    - Extend config to add initial variables of the smart contracts used as
      defaults before they are changed via events.
    - In stopped channels, set size 1 so that panics are not witheld until the
      node stops completely.
- common:
    - In Smart Contract variables, comment:
      - `WDelayerVariables.HermezRollupAddress` because it's not needed.
      - `RollupVariables.Buckets` because there are no events for it, and for
        now it's not used.
- historydb:
    - Add functions to get and set smart contract variables.
- db:
    - Add `Rollback` function in `utils.go` to reduce boilerplate in sql
      transaction rollbacks in defers in db functions.
    - Update `rollup_vars` and `auction_vars` (renamed from `consensus_vars`)
      table, and add `wdelayer_vars` table.
- synchronizer:
    - Synchronize WDelayer
    - Handle SC variables properly
- test/ethclient:
    - Add essential implementation of WDelayer

config/config.go

@@ -8,6 +8,7 @@ import (
 	"github.com/BurntSushi/toml"
 	ethCommon "github.com/ethereum/go-ethereum/common"
 	"github.com/hermeznetwork/hermez-node/common"
+	"github.com/hermeznetwork/hermez-node/synchronizer"
 	"gopkg.in/go-playground/validator.v9"
 )
 
@@ -65,16 +66,14 @@ type Node struct {
 		URL string `validate:"required"`
 	} `validate:"required"`
 	Synchronizer struct {
-		SyncLoopInterval Duration `validate:"required"`
-		StartBlockNum    struct {
-			Rollup   int64 `validate:"required"`
-			Auction  int64 `validate:"required"`
-			WDelayer int64 `validate:"required"`
-		} `validate:"required"`
+		SyncLoopInterval Duration                         `validate:"required"`
+		StartBlockNum    synchronizer.ConfigStartBlockNum `validate:"required"`
+		InitialVariables synchronizer.SCVariables         `validate:"required"`
 	} `validate:"required"`
 	SmartContracts struct {
 		Rollup       ethCommon.Address `validate:"required"`
 		Auction      ethCommon.Address `validate:"required"`
+		WDelayer     ethCommon.Address `validate:"required"`
 		TokenHEZ     ethCommon.Address `validate:"required"`
 		TokenHEZName string            `validate:"required"`
 	} `validate:"required"`