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.
if (*block_height % 100 == 0 && progress_end - progress_begin > 0.0) {
                ShowProgress(strprintf("%s " + _("Rescanning..."), GetDisplayName()), std::max(1, std::min(99, (int)((progress_current - progress_begin) / (progress_end - progress_begin) * 100))));
            }
            if (GetTime() >= nNow + 60) {
                nNow = GetTime();
                WalletLogPrintf("Still rescanning. At block %d. Progress=%f\n", *block_height, progress_current);
            }
            CBlock block;
            if (chain().findBlock(block_hash, &block) && !block.IsNull()) {
                auto locked_chain = chain().lock();
                LOCK(cs_wallet);
                if (!locked_chain->getBlockHeight(block_hash)) {
                    
                    
                    
                    
                    result.last_failed_block = block_hash;
                    result.status = ScanResult::FAILURE;
                    break;
                }
                for (size_t posInBlock = 0; posInBlock < block.vtx.size(); ++posInBlock) {
                    SyncTransaction(block.vtx[posInBlock], block_hash, posInBlock, fUpdate);
                }
                
                result.last_scanned_block = block_hash;
                result.last_scanned_height = *block_height;
            } else {
                
                result.last_failed_block = block_hash;
                result.status = ScanResult::FAILURE;