Fix exit table, set delayed_withdrawn in exits

- In exit table, `instant_withdrawn`, `delayed_withdraw_request`, and
  `delayed_withdrawn` were referencing batch_num.  But these actions happen
  outside a batch, so they should reference a block_num.
- Process delayed withdrawns:
    - In Synchronizer, first match a Rollup delayed withdrawn request, with the
      WDelayer deposit (via TxHash), and store the owner and token associated
      with the delayed withdrawn.
    - In HistoryDB: store the owner and token of a delayed withdrawal request
      in the exit_tree, and set delayed_withdrawn when the withdraw is done in
      the WDelayer.
- Update dependency of sqlx to master
    - Last release of sqlx is from 2018 October, and it doesn't support
      `NamedQuery` with a slice of structs, which is used in this commit.

db/l2db/l2db.go

@@ -235,7 +235,7 @@ func (l2db *L2DB) InvalidateTxs(txIDs []common.TxID, batchNum common.BatchNum) e
 // CheckNonces invalidate txs with nonces that are smaller or equal than their respective accounts nonces.
 // The state of the affected txs will be changed from Pending -> Invalid
 func (l2db *L2DB) CheckNonces(updatedAccounts []common.Account, batchNum common.BatchNum) (err error) {
-	txn, err := l2db.db.Begin()
+	txn, err := l2db.db.Beginx()
 	if err != nil {
 		return err
 	}
@@ -280,7 +280,7 @@ func (l2db *L2DB) Reorg(lastValidBatch common.BatchNum) error {
 // Purge deletes transactions that have been forged or marked as invalid for longer than the safety period
 // it also deletes txs that has been in the L2DB for longer than the ttl if maxTxs has been exceeded
 func (l2db *L2DB) Purge(currentBatchNum common.BatchNum) (err error) {
-	txn, err := l2db.db.Begin()
+	txn, err := l2db.db.Beginx()
 	if err != nil {
 		return err
 	}