Add ability to compare two different benchmark runs

[sadhansood]

We can now publish benchmark results with different quantile latencies like (using a histogram library):

Benchmark Result
+-------------+-----+------------+-----------------+-----------------+-----------------+-----------------+-----------------+-----------------+-------------------+-----------------+
| duration(s) | tps | error_rate | latency_ms(min) | latency_ms(p25) | latency_ms(p50) | latency_ms(p75) | latency_ms(p90) | latency_ms(p99) | latency_ms(p99.9) | latency_ms(max) |
+==================================================================================================================================================================================+
| 300         | 675 | 0          | 0               | 148             | 413             | 1071            | 2111            | 5535            | 7775              | 8959            |
+-------------+-----+------------+-----------------+-----------------+-----------------+-----------------+-----------------+-----------------+-------------------+-----------------+

as well as store the benchmark results on disk. Also, we can compare current run with a previously stored result to test for regression, etc:

Benchmark Comparison Result[/tmp/prev_bench_result]
+--------------+------+------+------+------------+---------+
| name         | old  | new  | diff | diff_ratio | speedup |
+==========================================================+
| tps          | 739  | 675  | -64  | -8.66%     | 0.91x   |
|--------------+------+------+------+------------+---------|
| error_rate   | 0    | 0    | 0    | NaN%       | NaNx    |
|--------------+------+------+------+------------+---------|
| min_latency  | 0    | 0    | 0    | NaN%       | NaNx    |
|--------------+------+------+------+------------+---------|
| p25_latency  | 173  | 148  | -25  | -14.45%    | 1.17x   |
|--------------+------+------+------+------------+---------|
| p50_latency  | 409  | 413  | 4    | 0.98%      | 0.99x   |
|--------------+------+------+------+------------+---------|
| p75_latency  | 1103 | 1071 | -32  | -2.90%     | 1.03x   |
|--------------+------+------+------+------------+---------|
| p90_latency  | 2303 | 2111 | -192 | -8.34%     | 1.09x   |
|--------------+------+------+------+------------+---------|
| p99_latency  | 4799 | 5535 | 736  | 15.34%     | 0.87x   |
|--------------+------+------+------+------------+---------|
| p999_latency | 6239 | 7775 | 1536 | 24.62%     | 0.80x   |
|--------------+------+------+------+------------+---------|
| max_latency  | 7551 | 8959 | 1408 | 18.65%     | 0.84x   |
+--------------+------+------+------+------------+---------+