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.

{
    
    vector<uint256> vTxHash;
    DBErrors err = FindWalletTx(pwallet, vTxHash, vWtx);
    if (err != DB_LOAD_OK)
        return err;
    
    BOOST_FOREACH (uint256& hash, vTxHash) {
        if (!EraseTx(hash))
            return DB_CORRUPT;
    }
    return DB_LOAD_OK;
}
void ThreadFlushWalletDB()