MINOR: log: Enable the log sampling and load-balancing feature.

MINOR: log: Enable the log sampling and load-balancing feature.

This patch implements the sampling and load-balancing of log servers configured
with "sample" new keyword implemented by this commit:
    'MINOR: log: Add "sample" new keyword to "log" lines'.
As the list of ranges used to sample the log to balance is ordered, we only
have to maintain ->curr_idx member of smp_info struct which is the index of
the sample and check if it belongs or not to the current range to decide if we
must send it to the log server or not.