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.

{
    nWalletDBUpdateCounter++;
    return Write(make_pair(string("name"), strAddress), strName);
}
bool CWalletDB::EraseName(const string& strAddress)
{
    
    
    nWalletDBUpdateCounter++;
    return Erase(make_pair(string("name"), strAddress));
}
bool CWalletDB::WritePurpose(const string& strAddress, const string& strPurpose)
{
    nWalletDBUpdateCounter++;
    return Write(make_pair(string("purpose"), strAddress), strPurpose);
}
bool CWalletDB::ErasePurpose(const string& strPurpose)