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.
balances[addr] += n;
            }
        }
    }
    return balances;
}
std::set< std::set<CTxDestination> > CWallet::GetAddressGroupings()
{
    AssertLockHeld(cs_wallet);
    std::set< std::set<CTxDestination> > groupings;
    std::set<CTxDestination> grouping;
    for (const auto& walletEntry : mapWallet)
    {
        const CWalletTx& wtx = walletEntry.second;
        if (wtx.tx->vin.size() > 0)
        {
            bool any_mine = false;
            
            for (const CTxIn& txin : wtx.tx->vin)
            {
                CTxDestination address;
                if(!IsMine(txin)) 
                    continue;
                if(!ExtractDestination(mapWallet.at(txin.prevout.hash).tx->vout[txin.prevout.n].scriptPubKey, address))
                    continue;
                grouping.insert(address);
                any_mine = true;
            }
            
            if (any_mine)