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.
g_connNotifyEntryRemoved.erase(&pool);
}
CMainSignals& GetMainSignals()
{
    return g_signals;
}
void RegisterValidationInterface(CValidationInterface* pwalletIn) {
    ValidationInterfaceConnections& conns = g_signals.m_internals->m_connMainSignals[pwalletIn];
    conns.UpdatedBlockTip = g_signals.m_internals->UpdatedBlockTip.connect(std::bind(&CValidationInterface::UpdatedBlockTip, pwalletIn, std::placeholders::_1, std::placeholders::_2, std::placeholders::_3));
    conns.TransactionAddedToMempool = g_signals.m_internals->TransactionAddedToMempool.connect(std::bind(&CValidationInterface::TransactionAddedToMempool, pwalletIn, std::placeholders::_1));
    conns.BlockConnected = g_signals.m_internals->BlockConnected.connect(std::bind(&CValidationInterface::BlockConnected, pwalletIn, std::placeholders::_1, std::placeholders::_2, std::placeholders::_3));
    conns.BlockDisconnected = g_signals.m_internals->BlockDisconnected.connect(std::bind(&CValidationInterface::BlockDisconnected, pwalletIn, std::placeholders::_1));
    conns.TransactionRemovedFromMempool = g_signals.m_internals->TransactionRemovedFromMempool.connect(std::bind(&CValidationInterface::TransactionRemovedFromMempool, pwalletIn, std::placeholders::_1));
    conns.ChainStateFlushed = g_signals.m_internals->ChainStateFlushed.connect(std::bind(&CValidationInterface::ChainStateFlushed, pwalletIn, std::placeholders::_1));
    conns.BlockChecked = g_signals.m_internals->BlockChecked.connect(std::bind(&CValidationInterface::BlockChecked, pwalletIn, std::placeholders::_1, std::placeholders::_2));
    conns.NewPoWValidBlock = g_signals.m_internals->NewPoWValidBlock.connect(std::bind(&CValidationInterface::NewPoWValidBlock, pwalletIn, std::placeholders::_1, std::placeholders::_2));
}
void UnregisterValidationInterface(CValidationInterface* pwalletIn) {
    if (g_signals.m_internals) {
        g_signals.m_internals->m_connMainSignals.erase(pwalletIn);
    }
}
void UnregisterAllValidationInterfaces() {
    if (!g_signals.m_internals) {
        return;
    }
    g_signals.m_internals->m_connMainSignals.clear();
}
void CallFunctionInValidationInterfaceQueue(std::function<void ()> func) {