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.
{
    int64_t nNow = GetTime();
    assert(reserver.isReserved());
    uint256 block_hash = start_block;
    ScanResult result;
    WalletLogPrintf("Rescan started from block %s...\n", start_block.ToString());
    {
        fAbortRescan = false;
        ShowProgress(strprintf("%s " + _("Rescanning..."), GetDisplayName()), 0); 
        uint256 tip_hash;
        
        Optional<int> block_height = MakeOptional(false, int());
        double progress_begin;
        double progress_end;
        {
            auto locked_chain = chain().lock();
            if (Optional<int> tip_height = locked_chain->getHeight()) {
                tip_hash = locked_chain->getBlockHash(*tip_height);
            }
            block_height = locked_chain->getBlockHeight(block_hash);
            progress_begin = chain().guessVerificationProgress(block_hash);
            progress_end = chain().guessVerificationProgress(stop_block.IsNull() ? tip_hash : stop_block);
        }
        double progress_current = progress_begin;
        while (block_height && !fAbortRescan && !chain().shutdownRequested()) {