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 (fFileBacked)
    {
        CWalletDB walletdb(strWalletFile);
        walletdb.ErasePool(nIndex);
    }
    LogPrintf("keypool keep %d\n", nIndex);
}
void CWallet::ReturnKey(int64_t nIndex)