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 <boost/thread.hpp>
extern CWallet* pwalletMain;
/**
 * Settings
 */
extern CFeeRate payTxFee;
extern unsigned int nTxConfirmTarget;
extern bool bSpendZeroConfChange;
extern bool fSendFreeTransactions;
extern bool fWalletRbf;
static const unsigned int DEFAULT_KEYPOOL_SIZE = 100;
static const CAmount DEFAULT_TRANSACTION_FEE = 0;
static const CAmount DEFAULT_FALLBACK_FEE = 20000;
static const CAmount DEFAULT_TRANSACTION_MINFEE = 1000;
static const CAmount WALLET_INCREMENTAL_RELAY_FEE = 5000;
static const CAmount MIN_CHANGE = CENT;
static const CAmount MIN_FINAL_CHANGE = MIN_CHANGE/2;
static const bool DEFAULT_SPEND_ZEROCONF_CHANGE = true;
static const bool DEFAULT_SEND_FREE_TRANSACTIONS = false;
static const bool DEFAULT_WALLET_REJECT_LONG_CHAINS = false;
static const unsigned int DEFAULT_TX_CONFIRM_TARGET = 6;
static const bool DEFAULT_WALLET_RBF = false;
static const unsigned int MAX_FREE_TRANSACTION_CREATE_SIZE = 1000;
static const bool DEFAULT_WALLETBROADCAST = true;
static const bool DEFAULT_DISABLE_WALLET = false;
static const bool DEFAULT_USE_HD_WALLET = true;
extern const char * DEFAULT_WALLET_DAT;
class CBlockIndex;
class CCoinControl;
class COutput;