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.
headers[0].hashPrevBlock.ToString(),
                    pindexBestHeader->nHeight,
                    pfrom->GetId(), nodestate->nUnconnectingHeaders);
            
            
            
            UpdateBlockAvailability(pfrom->GetId(), headers.back().GetHash());
            if (nodestate->nUnconnectingHeaders % MAX_UNCONNECTING_HEADERS == 0) {
                Misbehaving(pfrom->GetId(), 20);
            }
            return true;
        }
        uint256 hashLastBlock;
        for (const CBlockHeader& header : headers) {
            if (!hashLastBlock.IsNull() && header.hashPrevBlock != hashLastBlock) {
                Misbehaving(pfrom->GetId(), 20, "non-continuous headers sequence");
                return false;
            }
            hashLastBlock = header.GetHash();
        }
        
        
        if (!LookupBlockIndex(hashLastBlock)) {
            received_new_header = true;
        }
    }
    CValidationState state;
    CBlockHeader first_invalid_header;
    if (!ProcessNewBlockHeaders(headers, state, chainparams, &pindexLast, &first_invalid_header)) {
        int nDoS;