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 (!meta.hd_seed_id.IsNull() && !meta.has_key_origin && meta.hdKeypath != "s") { 
            CKey key;
            GetKey(meta.hd_seed_id, key);
            CExtKey masterKey;
            masterKey.SetSeed(key.begin(), key.size());
            
            CKeyID master_id = masterKey.key.GetPubKey().GetID();
            std::copy(master_id.begin(), master_id.begin() + 4, meta.key_origin.fingerprint);
            if (!ParseHDKeypath(meta.hdKeypath, meta.key_origin.path)) {
                throw std::runtime_error("Invalid stored hdKeypath");
            }
            meta.has_key_origin = true;
            if (meta.nVersion < CKeyMetadata::VERSION_WITH_KEY_ORIGIN) {
                meta.nVersion = CKeyMetadata::VERSION_WITH_KEY_ORIGIN;
            }
            
            CPubKey pubkey;
            if (GetPubKey(meta_pair.first, pubkey)) {
                batch->WriteKeyMetadata(meta, pubkey, true);
                if (++cnt % 1000 == 0) {
                    
                    batch.reset(new WalletBatch(*database));
                }
            }
        }
    }
    batch.reset(); 
    SetWalletFlag(WALLET_FLAG_KEY_ORIGIN_METADATA);
}
bool CWallet::LoadCryptedKey(const CPubKey &vchPubKey, const std::vector<unsigned char> &vchCryptedSecret)