[move] Move PoissonNextSend to src/random and update comment

PoissonNextSend is used by net and net_processing and is stateless, so
place it in the utility random.cpp translation unit.

src/random.h

@@ -10,7 +10,7 @@
 #include <crypto/common.h>
 #include <uint256.h>
 
-#include <chrono> // For std::chrono::microseconds
+#include <chrono>
 #include <cstdint>
 #include <limits>
 
@@ -82,6 +82,18 @@ D GetRandomDuration(typename std::common_type<D>::type max) noexcept
 };
 constexpr auto GetRandMicros = GetRandomDuration<std::chrono::microseconds>;
 constexpr auto GetRandMillis = GetRandomDuration<std::chrono::milliseconds>;
+
+/**
+ * Return a timestamp in the future sampled from an exponential distribution
+ * (https://en.wikipedia.org/wiki/Exponential_distribution). This distribution
+ * is memoryless and should be used for repeated network events (e.g. sending a
+ * certain type of message) to minimize leaking information to observers.
+ *
+ * The probability of an event occuring before time x is 1 - e^-(x/a) where a
+ * is the average interval between events.
+ * */
+std::chrono::microseconds PoissonNextSend(std::chrono::microseconds now, std::chrono::seconds average_interval);
+
 int GetRandInt(int nMax) noexcept;
 uint256 GetRandHash() noexcept;