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 Erase(make_pair(string("purpose"), strPurpose));
}
bool CWalletDB::WriteTx(const CWalletTx& wtx)
{
    nWalletDBUpdateCounter++;
    return Write(std::make_pair(std::string("tx"), wtx.GetHash()), wtx);
}
bool CWalletDB::EraseTx(uint256 hash)
{
    nWalletDBUpdateCounter++;
    return Erase(std::make_pair(std::string("tx"), hash));
}
bool CWalletDB::WriteKey(const CPubKey& vchPubKey, const CPrivKey& vchPrivKey, const CKeyMetadata& keyMeta)
{
    nWalletDBUpdateCounter++;
    if (!Write(std::make_pair(std::string("keymeta"), vchPubKey),
               keyMeta, false))
        return false;
    
    std::vector<unsigned char> vchKey;
    vchKey.reserve(vchPubKey.size() + vchPrivKey.size());
    vchKey.insert(vchKey.end(), vchPubKey.begin(), vchPubKey.end());
    vchKey.insert(vchKey.end(), vchPrivKey.begin(), vchPrivKey.end());
    return Write(std::make_pair(std::string("key"), vchPubKey), std::make_pair(vchPrivKey, Hash(vchKey.begin(), vchKey.end())), false);
}
bool CWalletDB::WriteCryptedKey(const CPubKey& vchPubKey,