MINOR: log: remove the unused curr_idx in struct smp_log_range

MINOR: log: remove the unused curr_idx in struct smp_log_range

This index is useless because it only serves to know when the global
index reached the end, while the global one already knows it. Let's
just drop it and perform the test on the global range.

It was verified with the following config that the first server continues
to take 1/10 of the traffic, the 2nd one 2/10, the 3rd one 3/10 and the
4th one 4/10:

    log 127.0.0.1:10001 sample 1:10 local0
    log 127.0.0.1:10002 sample 2,5:10 local0
    log 127.0.0.1:10003 sample 3,7,9:10 local0
    log 127.0.0.1:10004 sample 4,6,8,10:10 local0