Merge #17243: p2p: add PoissonNextSend method that returns mockable time

1a8f0d5a74 [tools] update nNextInvSend to use mockable time (Amiti Uttarwar)
4de630354f [tools] add PoissonNextSend method that returns mockable time (Amiti Uttarwar)

Pull request description:

  Introduce a Poisson helper method that wraps the existing method to return `std::chrono::duration` type, which is mockable.

  Needed for https://github.com/bitcoin/bitcoin/pull/16698.

ACKs for top commit:
  ajtowns:
    ACK 1a8f0d5a74
  MarcoFalke:
    re-ACK 1a8f0d5a74
  naumenkogs:
    ACK 1a8f0d5, and let's merge it and come back to it later.

Tree-SHA512: 7e2325d7c55fc0b4357cb86b83e0c218ba269f678c1786342d8bc380bfd9696373bc24ff124b9ff17a6e761c62b2b44ff5247c3911e2afdc7cc5c20417e8290b

src/net.h

@@ -808,7 +808,7 @@ public:
         bool fSendMempool GUARDED_BY(cs_tx_inventory){false};
         // Last time a "MEMPOOL" request was serviced.
         std::atomic<std::chrono::seconds> m_last_mempool_req{std::chrono::seconds{0}};
-        int64_t nNextInvSend{0};
+        std::chrono::microseconds nNextInvSend{0};
 
         CCriticalSection cs_feeFilter;
         // Minimum fee rate with which to filter inv's to this node
@@ -991,11 +991,13 @@ public:
     void MaybeSetAddrName(const std::string& addrNameIn);
 };
 
-
-
-
-
 /** Return a timestamp in the future (in microseconds) for exponentially distributed events. */
 int64_t PoissonNextSend(int64_t now, int average_interval_seconds);
 
+/** Wrapper to return mockable type */
+inline std::chrono::microseconds PoissonNextSend(std::chrono::microseconds now, std::chrono::seconds average_interval)
+{
+    return std::chrono::microseconds{PoissonNextSend(now.count(), average_interval.count())};
+}
+
 #endif // BITCOIN_NET_H