db_bench readrandom value_size 1024B stuck at 72m ops

### Expected behavior
Would have finished at least.

### Actual behavior
It got stuck at 72300000 ops.
Ram and ssd still had much free space(ram 10g+, ssd 500g+), and cpu utilization was nearly 0. I guess deadlock occurred

### Steps to reproduce the behavior
version: last commit on Dec 15, 2018 a914a1c6dc37c63ac06186767235aa9d8f17469c
```
cmake -DCMAKE_BUILD_TYPE=Release -DWITH_SNAPPY=1 -DUSE_RTTI=1 ..
make db_bench
../Release/db_bench \
--benchmarks="fillrandom,readrandom" \
--db="/mnt/ssd" \
--key_size=16 \
--reads=100000 \
--write_buffer_size=`expr 64 \* 1048576` \
--max_bytes_for_level_base=`expr 256 \* 1048576` \
--use_existing_db=0 \
--max_background_compactions=16 \
--value_size=1024 \
--num=80000000
```

heres the LOG