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 "base58.h"
#include "clientversion.h"
#include "init.h"
#include "validation.h"
#include "net.h"
#include "netbase.h"
#include "rpc/server.h"
#include "timedata.h"
#include "util.h"
#include "utilstrencodings.h"
#ifdef ENABLE_WALLET
#include "wallet/wallet.h"
#include "wallet/walletdb.h"
#endif
#include <stdint.h>
#include <boost/assign/list_of.hpp>
#include <univalue.h>
using namespace std;
/**
 * @note Do not add or change anything in the information returned by this
 * method. `getinfo` exists for backwards-compatibility only. It combines
 * information from wildly different sources in the program, which is a mess,