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.
int nDoS = 0;
        if (state.IsInvalid(nDoS))
        {
            LogPrint(BCLog::MEMPOOLREJ, "%s from peer=%d was not accepted: %s\n", tx.GetHash().ToString(),
                pfrom->GetId(),
                FormatStateMessage(state));
            if (enable_bip61 && state.GetRejectCode() > 0 && state.GetRejectCode() < REJECT_INTERNAL) { 
                connman->PushMessage(pfrom, msgMaker.Make(NetMsgType::REJECT, strCommand, (unsigned char)state.GetRejectCode(),
                                   state.GetRejectReason().substr(0, MAX_REJECT_MESSAGE_LENGTH), inv.hash));
            }
            if (nDoS > 0) {
                Misbehaving(pfrom->GetId(), nDoS);
            }
        }
        return true;
    }
    if (strCommand == NetMsgType::CMPCTBLOCK)
    {
        
        if (fImporting || fReindex) {
            LogPrint(BCLog::NET, "Unexpected cmpctblock message received from peer %d\n", pfrom->GetId());
            return true;
        }
        CBlockHeaderAndShortTxIDs cmpctblock;
        vRecv >> cmpctblock;
        bool received_new_header = false;
        {
        LOCK(cs_main);
        if (!LookupBlockIndex(cmpctblock.header.hashPrevBlock)) {