When importing a watch-only address over `importmulti` with a specific timestamp, the wallet's `nTimeFirstKey` will be set to 1.
I think we should add a optional timestamp to `CWallet::AddWatchOnly()` and use it when importing addresses or pubkeys.

#ifndef BITCOIN_WALLET_WALLETDB_H
#define BITCOIN_WALLET_WALLETDB_H
#include "amount.h"
#include "primitives/transaction.h"
#include "wallet/db.h"
#include "key.h"
#include <list>
#include <stdint.h>
#include <string>
#include <utility>
#include <vector>
static const bool DEFAULT_FLUSHWALLET = true;
class CAccount;