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.
#include <util/moneystr.h>
#include <util/strencodings.h>
#include <memory>
#if defined(NDEBUG)
# error "Bitcoin cannot be compiled without assertions."
#endif
static constexpr int64_t ORPHAN_TX_EXPIRE_TIME = 20 * 60;
static constexpr int64_t ORPHAN_TX_EXPIRE_INTERVAL = 5 * 60;
/** Headers download timeout expressed in microseconds
 *  Timeout = base + per_header * (expected number of headers) */
static constexpr int64_t HEADERS_DOWNLOAD_TIMEOUT_BASE = 15 * 60 * 1000000; 
static constexpr int64_t HEADERS_DOWNLOAD_TIMEOUT_PER_HEADER = 1000; 
/** Protect at least this many outbound peers from disconnection due to slow/
 * behind headers chain.
 */
static constexpr int32_t MAX_OUTBOUND_PEERS_TO_PROTECT_FROM_DISCONNECT = 4;
static constexpr int64_t CHAIN_SYNC_TIMEOUT = 20 * 60; 
static constexpr int64_t STALE_CHECK_INTERVAL = 10 * 60; 
static constexpr int64_t EXTRA_PEER_CHECK_INTERVAL = 45;
static constexpr int64_t MINIMUM_CONNECT_TIME = 30;
static constexpr uint64_t RANDOMIZER_ID_ADDRESS_RELAY = 0x3cac0035b5866b90ULL;
static constexpr int STALE_RELAY_AGE_LIMIT = 30 * 24 * 60 * 60;
static constexpr int HISTORICAL_BLOCK_AGE = 7 * 24 * 60 * 60;
static constexpr int32_t MAX_PEER_TX_IN_FLIGHT = 100;
static constexpr int32_t MAX_PEER_TX_ANNOUNCEMENTS = 2 * MAX_INV_SZ;
static constexpr int64_t INBOUND_PEER_TX_DELAY = 2 * 1000000;
static constexpr int64_t GETDATA_TX_INTERVAL = 60 * 1000000;
static constexpr int64_t MAX_GETDATA_RANDOM_DELAY = 2 * 1000000;
static_assert(INBOUND_PEER_TX_DELAY >= MAX_GETDATA_RANDOM_DELAY,
"To preserve security, MAX_GETDATA_RANDOM_DELAY should not exceed INBOUND_PEER_DELAY");