osd_publish_batch : optimize the Fact loop - use const ref to avoid u…

[GoreProgrammer]

Since Fact contains FactMeta struct and an std:: variant field that in turn can contain a string, its copying is not an entirely trivial operation. We can avoid the extra Fact copying operations by using const reference in the loop:

for (const Fact& fact: *facts)

instead of the original

for (Fact fact : *facts)

Another option could be

for (Fact& fact : *facts) {                // Non-const ref
    fact_queue.push(std::move(fact));      // Move
}

Perhaps this one would be even better, but it requires some testing (which I cannot do at the moment) while the suggested change is 100% risk free.