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.

{
    
    
    
    
    
    
    
    int64_t now = GetTime();
    std::string newFilename = strprintf("wallet.%d.bak", now);
    int result = dbenv.dbenv->dbrename(NULL, filename.c_str(), NULL,
                                       newFilename.c_str(), DB_AUTO_COMMIT);
    if (result == 0)