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 (nIndex == -1)
    {
        CKeyPool keypool;
        pwallet->ReserveKeyFromKeyPool(nIndex, keypool);
        if (nIndex != -1)
            vchPubKey = keypool.vchPubKey;
        else {
            return false;
        }
    }
    assert(vchPubKey.IsValid());
    pubkey = vchPubKey;
    return true;
}
void CReserveKey::KeepKey()
{
    if (nIndex != -1)
        pwallet->KeepKey(nIndex);
    nIndex = -1;
    vchPubKey = CPubKey();
}
void CReserveKey::ReturnKey()