[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.cpp

@@ -19,6 +19,7 @@
 #include <sync.h>     // for Mutex
 #include <util/time.h> // for GetTimeMicros()
 
+#include <cmath>
 #include <stdlib.h>
 #include <thread>
 
@@ -714,3 +715,9 @@ void RandomInit()
 
     ReportHardwareRand();
 }
+
+std::chrono::microseconds PoissonNextSend(std::chrono::microseconds now, std::chrono::seconds average_interval)
+{
+    double unscaled = -log1p(GetRand(1ULL << 48) * -0.0000000000000035527136788 /* -1/2^48 */);
+    return now + std::chrono::duration_cast<std::chrono::microseconds>(unscaled * average_interval + 0.5us);
+}