The `Broadcast`/`ResendWalletTransactions` interface is strange:

- it's the node saying to the wallet "Try to submit your unconfirmed transactions to the mempool again"
- it's called in one place by the node, in `SendMessages()` here: https://github.com/bitcoin/bitcoin/blob/c033c4b5cef89a654e4d9d5c5f9bd823871b068b/src/net_processing.cpp#L3515. `SendMessages()` is called on the message handling thread on a loop.
- the actual scheduling of when the wallet sends messages is handled in the `ResendWalletTransactions()` function itself, using the `nNextResend` and `nLastResend` wallet globals (which could as easily be static variables in `ResendWalletTransactions()`).

Therefore the only use of `Broadcast()` is to periodically prod the wallet. The wallet itself decides whether it's time to resend transactions. This could be done by scheduling regular calls to `ResendWalletTransactions()`, in the same way that we schedule regular wallet flushes here: https://github.com/bitcoin/bitcoin/blob/c033c4b5cef89a654e4d9d5c5f9bd823871b068b/src/wallet/init.cpp#L215.

A couple of minor implementation details:

- the call to `Broadcast()` in `SendMessages()` is protected by an if statement `if (!fReindex && !fImporting && !IsInitialBlockDownload()) {`. Those tests could be moved into `ResendWalletTransactions()` (through the `Chain` interface).
- The `Broadcast` notification passes in the `nTimeBestReceived` time of the best block. That could be fetched by `ResendWalletTransactions()` through the `Chain` interface.
void Serialize(Stream& s) const
    {
        char fSpent = false;
        mapValue_t mapValueCopy = mapValue;
        mapValueCopy["fromaccount"] = "";
        WriteOrderPos(nOrderPos, mapValueCopy);
        if (nTimeSmart) {
            mapValueCopy["timesmart"] = strprintf("%u", nTimeSmart);
        }
        s << static_cast<const CMerkleTx&>(*this);
        std::vector<CMerkleTx> vUnused; 
        s << vUnused << mapValueCopy << vOrderForm << fTimeReceivedIsTxTime << nTimeReceived << fFromMe << fSpent;
    }
    template<typename Stream>
    void Unserialize(Stream& s)
    {
        Init(nullptr);
        char fSpent;
        s >> static_cast<CMerkleTx&>(*this);
        std::vector<CMerkleTx> vUnused; 
        s >> vUnused >> mapValue >> vOrderForm >> fTimeReceivedIsTxTime >> nTimeReceived >> fFromMe >> fSpent;
        ReadOrderPos(nOrderPos, mapValue);
        nTimeSmart = mapValue.count("timesmart") ? (unsigned int)atoi64(mapValue["timesmart"]) : 0;
        mapValue.erase("fromaccount");
        mapValue.erase("spent");
        mapValue.erase("n");
        mapValue.erase("timesmart");
    }
    
    void MarkDirty()