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.
{
            LOCK(pfrom->cs_filter);
            pfrom->pfilter.reset(new CBloomFilter(filter));
            pfrom->pfilter->UpdateEmptyFull();
            pfrom->fRelayTxes = true;
        }
        return true;
    }
    if (strCommand == NetMsgType::FILTERADD) {
        std::vector<unsigned char> vData;
        vRecv >> vData;
        
        
        bool bad = false;
        if (vData.size() > MAX_SCRIPT_ELEMENT_SIZE) {
            bad = true;
        } else {
            LOCK(pfrom->cs_filter);
            if (pfrom->pfilter) {
                pfrom->pfilter->insert(vData);
            } else {
                bad = true;
            }
        }
        if (bad) {
            LOCK(cs_main);
            Misbehaving(pfrom->GetId(), 100);
        }
        return true;
    }