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.
else if (inv.type == MSG_FILTERED_BLOCK)
            {
                bool sendMerkleBlock = false;
                CMerkleBlock merkleBlock;
                {
                    LOCK(pfrom->cs_filter);
                    if (pfrom->pfilter) {
                        sendMerkleBlock = true;
                        merkleBlock = CMerkleBlock(*pblock, *pfrom->pfilter);
                    }
                }
                if (sendMerkleBlock) {
                    connman->PushMessage(pfrom, msgMaker.Make(NetMsgType::MERKLEBLOCK, merkleBlock));
                    
                    
                    
                    
                    
                    
                    typedef std::pair<unsigned int, uint256> PairType;
                    for (PairType& pair : merkleBlock.vMatchedTxn)
                        connman->PushMessage(pfrom, msgMaker.Make(SERIALIZE_TRANSACTION_NO_WITNESS, NetMsgType::TX, *pblock->vtx[pair.first]));
                }
                
                    
            }
            else if (inv.type == MSG_CMPCT_BLOCK)
            {