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_WALLET_H
#define BITCOIN_WALLET_WALLET_H
#include "amount.h"
#include "streams.h"
#include "tinyformat.h"
#include "ui_interface.h"
#include "utilstrencodings.h"
#include "validationinterface.h"
#include "script/ismine.h"
#include "script/sign.h"
#include "wallet/crypter.h"
#include "wallet/walletdb.h"
#include "wallet/rpcwallet.h"
#include <algorithm>
#include <atomic>
#include <map>
#include <set>
#include <stdexcept>
#include <stdint.h>
#include <string>
#include <utility>
#include <vector>
#include <boost/shared_ptr.hpp>