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.
* externally and came in through the network or sendrawtransaction RPC.
     */
    char fFromMe;
    int64_t nOrderPos; 
    std::multimap<int64_t, CWalletTx*>::const_iterator m_it_wtxOrdered;
    
    mutable bool fDebitCached;
    mutable bool fCreditCached;
    mutable bool fImmatureCreditCached;
    mutable bool fAvailableCreditCached;
    mutable bool fWatchDebitCached;
    mutable bool fWatchCreditCached;
    mutable bool fImmatureWatchCreditCached;
    mutable bool fAvailableWatchCreditCached;
    mutable bool fChangeCached;
    mutable bool fInMempool;
    mutable CAmount nDebitCached;
    mutable CAmount nCreditCached;
    mutable CAmount nImmatureCreditCached;
    mutable CAmount nAvailableCreditCached;
    mutable CAmount nWatchDebitCached;
    mutable CAmount nWatchCreditCached;
    mutable CAmount nImmatureWatchCreditCached;
    mutable CAmount nAvailableWatchCreditCached;
    mutable CAmount nChangeCached;
    CWalletTx(const CWallet* pwalletIn, CTransactionRef arg) : CMerkleTx(std::move(arg))
    {
        Init(pwalletIn);
    }
    void Init(const CWallet* pwalletIn)