[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/net.h

@@ -1270,9 +1270,6 @@ private:
     friend struct ConnmanTestMsg;
 };
 
-/** Return a timestamp in the future (in microseconds) for exponentially distributed events. */
-std::chrono::microseconds PoissonNextSend(std::chrono::microseconds now, std::chrono::seconds average_interval);
-
 /** Dump binary message to file, with timestamp */
 void CaptureMessage(const CAddress& addr, const std::string& msg_type, const Span<const unsigned char>& data, bool is_incoming);