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 missingInternal = std::max(std::max((int64_t) nTargetSize, (int64_t) 1) - (int64_t)setInternalKeyPool.size(), (int64_t) 0);
        if (!IsHDEnabled() || !CanSupportFeature(FEATURE_HD_SPLIT))
        {
            
            missingInternal = 0;
        }
        bool internal = false;
        WalletBatch batch(*database);
        for (int64_t i = missingInternal + missingExternal; i--;)
        {
            if (i < missingInternal) {
                internal = true;
            }
            CPubKey pubkey(GenerateNewKey(batch, internal));
            AddKeypoolPubkeyWithDB(pubkey, internal, batch);
        }
        if (missingInternal + missingExternal > 0) {
            WalletLogPrintf("keypool added %d keys (%d internal), size=%u (%u internal)\n", missingInternal + missingExternal, missingInternal, setInternalKeyPool.size() + setExternalKeyPool.size() + set_pre_split_keypool.size(), setInternalKeyPool.size());
        }
    }
    NotifyCanGetAddressesChanged();
    return true;
}
void CWallet::AddKeypoolPubkey(const CPubKey& pubkey, const bool internal)
{
    WalletBatch batch(*database);
    AddKeypoolPubkeyWithDB(pubkey, internal, batch);
    NotifyCanGetAddressesChanged();
}
void CWallet::AddKeypoolPubkeyWithDB(const CPubKey& pubkey, const bool internal, WalletBatch& batch)