Revise getRandom facilities

Use one of the following to provide random bytes:
- Windows CryptGenRandom
- Linux getrandom (syscall interface to urandom, without nasty corner
  cases such as file descriptor exhaustion or re-linked /dev/urandom)
- std::device_random (C++ random device, which usually will be urandom)

This also equalizes util::getRandom and SimpleRandomizer (the former
will now use the latter) instead of having essentially two different
PRNG interfaces with potentially different quality.