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.

#include "wallet/walletdb.h"
#include "base58.h"
#include "consensus/validation.h"
#include "validation.h" 
#include "protocol.h"
#include "serialize.h"
#include "sync.h"
#include "util.h"
#include "utiltime.h"
#include "wallet/wallet.h"
#include <atomic>
#include <boost/version.hpp>
#include <boost/filesystem.hpp>
#include <boost/foreach.hpp>
#include <boost/thread.hpp>
using namespace std;
static uint64_t nAccountingEntryNumber = 0;
static std::atomic<unsigned int> nWalletDBUpdateCounter;
bool CWalletDB::WriteName(const string& strAddress, const string& strName)