txmempool: Make entry time type-safe (std::chrono)

src/net.h

@@ -761,7 +761,7 @@ public:
         // Used for BIP35 mempool sending
         bool fSendMempool GUARDED_BY(cs_tx_inventory){false};
         // Last time a "MEMPOOL" request was serviced.
-        std::atomic<int64_t> timeLastMempoolReq{0};
+        std::atomic<std::chrono::seconds> m_last_mempool_req{std::chrono::seconds{0}};
         int64_t nNextInvSend{0};
 
         CCriticalSection cs_feeFilter;