add GetRandBytes() as wrapper for RAND_bytes()

- add a small wrapper in util around RAND_bytes() and replace with
  GetRandBytes() in the code to log errors from calling RAND_bytes()
- remove OpenSSL header rand.h where no longer needed
- move rand functions from util to new random.h/.cpp
We could use ``OPENSSL_cleanse()`` here and remove the cstring include I guess. Not sure if a secure erase is needed here, but we also do that when adding the performance data as entropy source.