Fix int_latency calculation for infinite timeout

[Dmitriy Kargapolov (x0id)]

Before the fix:

int_latency_base = now + (message_timeout_ms * 1000);
rd_avg_add(&rkb->rkb_avg_int_latency, int_latency_base - rkm->rkm_ts_timeout);

When message_timeout_ms = 0:
int_latency_base = now + 0 = now
rkm_ts_timeout = INT64_MAX (for infinite timeout)
Result: now - INT64_MAX = massive negative number

After the fix:

rd_avg_add(&rkb->rkb_avg_int_latency, now - rkm->rkm_ts_enq);

Always works regardless of timeout setting:
now - rkm_ts_enq = current_time - enqueue_time = actual queue time

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