[Newcomer] Improve random.repy performance

[choksi81]

Our functions in random.repy are slow. On my moderately modern !MacBook Pro 8,2:

#elements Repy Python
 1000     17ms   6ms
10000     87ms  10ms
50000    600ms  33ms

That doesn't sound terrible at first, but given that we also run on low-power platforms such as Android and !RaspberryPi, the picture changes. Under load, we see runtimes 10 to 200 times as high (for both Repy and Python on a !RasPi). As a consequence, places that use the random.repy functions become prone to timing out. This includes the node manager and Affixes trying to advertise (see r7148, and know that in unrestricted mode, the list of connports has 5000 elements).

We should (1) replace our implementation of random_sample with a port of Python's random.sample, and (2) also port Python's random.shuffle which is quite a bit faster than randomly sampling all of the elements of a list.

[choksi81]

I addressed (2), random_shuffle was added in r7218.

[choksi81]

..and updated in r7318. The previous version used reversed() which is disallowed in Repy. Thanks Monzur for noticing!