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.
}
    if (strCommand == NetMsgType::TX) {
        
        
        if (!fRelayTxes && (!pfrom->fWhitelisted || !gArgs.GetBoolArg("-whitelistrelay", DEFAULT_WHITELISTRELAY)))
        {
            LogPrint(BCLog::NET, "transaction sent in violation of protocol peer=%d\n", pfrom->GetId());
            return true;
        }
        CTransactionRef ptx;
        vRecv >> ptx;
        const CTransaction& tx = *ptx;
        CInv inv(MSG_TX, tx.GetHash());
        pfrom->AddInventoryKnown(inv);
        LOCK2(cs_main, g_cs_orphans);
        bool fMissingInputs = false;
        CValidationState state;
        CNodeState* nodestate = State(pfrom->GetId());
        nodestate->m_tx_download.m_tx_announced.erase(inv.hash);
        nodestate->m_tx_download.m_tx_in_flight.erase(inv.hash);
        EraseTxRequest(inv.hash);
        std::list<CTransactionRef> lRemovedTxn;
        if (!AlreadyHave(inv) &&
            AcceptToMemoryPool(mempool, state, ptx, &fMissingInputs, &lRemovedTxn, false , 0 )) {
            mempool.check(pcoinsTip.get());
            RelayTransaction(tx, connman);
            for (unsigned int i = 0; i < tx.vout.size(); i++) {
                auto it_by_prev = mapOrphanTransactionsByPrev.find(COutPoint(inv.hash, i));
                if (it_by_prev != mapOrphanTransactionsByPrev.end()) {
                    for (const auto& elem : it_by_prev->second) {