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.

{
    AssertLockHeld(cs_wallet); 
    int64_t nRet = nOrderPosNext++;
    if (pwalletdb) {
        pwalletdb->WriteOrderPosNext(nOrderPosNext);
    } else {
        CWalletDB(strWalletFile).WriteOrderPosNext(nOrderPosNext);
    }
    return nRet;
}
bool CWallet::AccountMove(std::string strFrom, std::string strTo, CAmount nAmount, std::string strComment)