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.
vRecv >> nVersion >> nServiceInt >> nTime >> addrMe;
        nSendVersion = std::min(nVersion, PROTOCOL_VERSION);
        nServices = ServiceFlags(nServiceInt);
        if (!pfrom->fInbound)
        {
            connman->SetServices(pfrom->addr, nServices);
        }
        if (!pfrom->fInbound && !pfrom->fFeeler && !pfrom->m_manual_connection && !HasAllDesirableServiceFlags(nServices))
        {
            LogPrint(BCLog::NET, "peer=%d does not offer the expected services (%08x offered, %08x expected); disconnecting\n", pfrom->GetId(), nServices, GetDesirableServiceFlags(nServices));
            if (enable_bip61) {
                connman->PushMessage(pfrom, CNetMsgMaker(INIT_PROTO_VERSION).Make(NetMsgType::REJECT, strCommand, REJECT_NONSTANDARD,
                                   strprintf("Expected to offer services %08x", GetDesirableServiceFlags(nServices))));
            }
            pfrom->fDisconnect = true;
            return false;
        }
        if (nVersion < MIN_PEER_PROTO_VERSION) {
            
            LogPrint(BCLog::NET, "peer=%d using obsolete version %i; disconnecting\n", pfrom->GetId(), nVersion);
            if (enable_bip61) {
                connman->PushMessage(pfrom, CNetMsgMaker(INIT_PROTO_VERSION).Make(NetMsgType::REJECT, strCommand, REJECT_OBSOLETE,
                                   strprintf("Version must be %d or greater", MIN_PEER_PROTO_VERSION)));
            }
            pfrom->fDisconnect = true;
            return false;
        }
        if (!vRecv.empty())
            vRecv >> addrFrom >> nNonce;
        if (!vRecv.empty()) {