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.

{
    return CCryptoKeyStore::AddWatchOnly(dest);
}
bool CWallet::Unlock(const SecureString& strWalletPassphrase)
{
    CCrypter crypter;
    CKeyingMaterial vMasterKey;
    {
        LOCK(cs_wallet);
        BOOST_FOREACH(const MasterKeyMap::value_type& pMasterKey, mapMasterKeys)
        {
            if(!crypter.SetKeyFromPassphrase(strWalletPassphrase, pMasterKey.second.vchSalt, pMasterKey.second.nDeriveIterations, pMasterKey.second.nDerivationMethod))
                return false;
            if (!crypter.Decrypt(pMasterKey.second.vchCryptedKey, vMasterKey))
                continue; 
            if (CCryptoKeyStore::Unlock(vMasterKey))
                return true;
        }
    }
    return false;
}
bool CWallet::ChangeWalletPassphrase(const SecureString& strOldWalletPassphrase, const SecureString& strNewWalletPassphrase)
{
    bool fWasLocked = IsLocked();
    {
        LOCK(cs_wallet);
        Lock();
        CCrypter crypter;
        CKeyingMaterial vMasterKey;
        BOOST_FOREACH(MasterKeyMap::value_type& pMasterKey, mapMasterKeys)