summarize_current_second -> hdr_value_at_percentile takes large ammount of cpu cycles on large thread count

[filipecosta90]

with 45 threads, we see that summarizing the metrics, specifically for the the percentiles per second, takes a very large ammount of cpu cycles (in the bellow case > 80%).

./memtier_benchmark  -p 12000 --hide-histogram --pipeline=10 -c 168 -t 45 -d 1000 --key-maximum=9625503 --key-pattern=R:R --ratio=1:0  --test-time=1200 --run-count=2

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[filipecosta90]

for future reference, we've released 2.1.4 with the above fix