6.27 readrandom & seekrandom has 5%~7% regression compare to 6.14

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### Expected behavior
There is no regression.

### Actual behavior
There is 5%~7% regression.

### Steps to reproduce the behavior

time ./db_bench \
--write_buffer_size=$((64 * 1024 * 1024)) \
--threads=1 \
--bloom_bits=10 \
--cache_size=$((128 * 1024 * 1024)) \
--benchmarks=fillseq,readrandom \
--compression_type=lz4 \
--block_size=4096 \
--memtablerep=skip_list \
--compression_ratio=0.5 \
--key_size=30 \
--value_size=256 \
--num=10000000 \
--batch_size=100 \
--db=$db_path \
--use_existing_db=0

6.14 output
```
readrandom : 12.220 micros/op 81829 ops/sec; 22.3 MB/s (10000000 of 10000000 found)
real 3m3.939s
user 2m41.962s
sys 0m29.829s
```

6.27
```
readrandom : 14.344 micros/op 69714 ops/sec; 19.0 MB/s (10000000 of 10000000 found)
real 3m30.919s
user 2m59.485s
sys 0m34.378s
```