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 (!CCryptoKeyStore::AddWatchOnly(dest))
        return false;
    nTimeFirstKey = 1; 
    NotifyWatchonlyChanged(true);
    if (!fFileBacked)
        return true;
    return CWalletDB(strWalletFile).WriteWatchOnly(dest);
}
bool CWallet::RemoveWatchOnly(const CScript &dest)
{
    AssertLockHeld(cs_wallet);
    if (!CCryptoKeyStore::RemoveWatchOnly(dest))
        return false;
    if (!HaveWatchOnly())
        NotifyWatchonlyChanged(false);
    if (fFileBacked)
        if (!CWalletDB(strWalletFile).EraseWatchOnly(dest))
            return false;
    return true;
}
bool CWallet::LoadWatchOnly(const CScript &dest)