Revert LATENCY_HDR_SEC_SIGDIGTS to 2. Optimize summarize_current_second by using hdr_value_at_percentiles() instead of N x hdr_value_at_percentile().

[filipecosta90]

Fixes #294

It also updates the hdr histogram dependency to make usage of 2 optimizations we've added there quite a while ago:

• Added new API hdr_value_at_percentiles() as a means for redudant computation removal HdrHistogram/HdrHistogram_c#90
• hdr_value_at_percentile 30% on cpu time reduction: reuse common computed values within highest_equivalent_value/lowest_equivalent_value/hdr_median_equivalent_value calls on hdr_next_non_equivalent_value HdrHistogram/HdrHistogram_c#94

Altogether, we've reverted the regression introduced in v2.1.1 and also reduced by 22% the CPU usage for the same QPS (even a bit higher).

Sample benchmark command:

./memtier_benchmark --hide-histogram --pipeline=1 -c 100 -t 100 -d 1000 --key-maximum=9625503 --key-pattern=R:R --ratio=1:0  --test-time=60

CPU usage fix and improvement (red is this PR) :

Ops/sec fix and improvement (red is this PR) :