MINOR: log/balance: support for the "random" lb algorithm

MINOR: log/balance: support for the "random" lb algorithm

In this patch we add basic support for the random algorithm:

random algorithm picks a random server using the result of the
statistical_prng() function as if it was a hash key to then compute the
related server ID.

There is no support for the <draw> parameter (which is implemented for
tcp/http load-balancing), because we don't have the required metrics to
evaluate server's load in log backends for the moment. Plus it would add
more complexity to the __do_send_log_backend() function so we'll keep it
this way for now but this might be needed in the future.