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.
connman->PushMessage(pfrom, msgMaker.Make(NetMsgType::SENDHEADERS));
        }
        if (pfrom->nVersion >= SHORT_IDS_BLOCKS_VERSION) {
            
            
            
            
            
            bool fAnnounceUsingCMPCTBLOCK = false;
            uint64_t nCMPCTBLOCKVersion = 2;
            if (pfrom->GetLocalServices() & NODE_WITNESS)
                connman->PushMessage(pfrom, msgMaker.Make(NetMsgType::SENDCMPCT, fAnnounceUsingCMPCTBLOCK, nCMPCTBLOCKVersion));
            nCMPCTBLOCKVersion = 1;
            connman->PushMessage(pfrom, msgMaker.Make(NetMsgType::SENDCMPCT, fAnnounceUsingCMPCTBLOCK, nCMPCTBLOCKVersion));
        }
        pfrom->fSuccessfullyConnected = true;
        return true;
    }
    if (!pfrom->fSuccessfullyConnected) {
        
        LOCK(cs_main);
        Misbehaving(pfrom->GetId(), 1);
        return false;
    }
    if (strCommand == NetMsgType::ADDR) {
        std::vector<CAddress> vAddr;
        vRecv >> vAddr;