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.

if (!crypter.SetKeyFromPassphrase(strWalletPassphrase, kMasterKey.vchSalt, kMasterKey.nDeriveIterations, kMasterKey.nDerivationMethod))
        return false;
    if (!crypter.Encrypt(vMasterKey, kMasterKey.vchCryptedKey))
        return false;
    {
        LOCK(cs_wallet);
        mapMasterKeys[++nMasterKeyMaxID] = kMasterKey;
        if (fFileBacked)
        {
            assert(!pwalletdbEncryption);
            pwalletdbEncryption = new CWalletDB(strWalletFile);
            if (!pwalletdbEncryption->TxnBegin()) {
                delete pwalletdbEncryption;
                pwalletdbEncryption = NULL;
                return false;
            }
            pwalletdbEncryption->WriteMasterKey(nMasterKeyMaxID, kMasterKey);
        }
        if (!EncryptKeys(vMasterKey))
        {
            if (fFileBacked) {
                pwalletdbEncryption->TxnAbort();
                delete pwalletdbEncryption;
            }
            
            
            assert(false);
        }
        
        SetMinVersion(FEATURE_WALLETCRYPT, pwalletdbEncryption, true);