scripted-diff: replace PoissonNextSend with GetExponentialRand

This distribution is used for more than just the next inv send, so make
the name more generic.

Also rename to "exponential" to avoid the confusion that this is a
poisson distribution.

-BEGIN VERIFY SCRIPT-
ren() { sed -i "s/\<$1\>/$2/g" $(git grep -l "$1" ./src) ; }

ren  PoissonNextSend   GetExponentialRand
ren  "a poisson timer" "an exponential timer"
-END VERIFY SCRIPT-

src/random.cpp

@@ -716,7 +716,7 @@ void RandomInit()
     ReportHardwareRand();
 }
 
-std::chrono::microseconds PoissonNextSend(std::chrono::microseconds now, std::chrono::seconds average_interval)
+std::chrono::microseconds GetExponentialRand(std::chrono::microseconds now, std::chrono::seconds average_interval)
 {
     double unscaled = -std::log1p(GetRand(uint64_t{1} << 48) * -0.0000000000000035527136788 /* -1/2^48 */);
     return now + std::chrono::duration_cast<std::chrono::microseconds>(unscaled * average_interval + 0.5us);