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.

{
    LOCK2(cs_main, cs_wallet);
    CWalletDB walletdb(strWalletFile, "r+");
    std::set<uint256> todo;
    std::set<uint256> done;
    
    assert(mapWallet.count(hashTx));
    CWalletTx& origtx = mapWallet[hashTx];
    if (origtx.GetDepthInMainChain() > 0 || origtx.InMempool()) {
        return false;
    }