Breaking build - Optimize leveldb block seeks to utilize the current iterator location

Hi,
I was trying to build LevelDB with the updates to master and noticed that my build is breaking.
On a quick look, I noticed that there was a commit made a couple of hours ago and that might be the reason for the breaking build.
Here is the link to that [commit](https://github.com/google/leveldb/commit/8cce47e450b365347769959c53b8836ef0216df9)
Here's the failure log from my machine (Ubuntu 20.04.1 LTS):
```
/home/odesai/leveldb/benchmarks/db_bench.cc:152:40: error: use of deleted function ‘std::atomic::atomic(const std::atomic&)’
152 | mutable std::atomic count_ = 0;
| ^
In file included from /home/odesai/leveldb/benchmarks/db_bench.cc:7:
/usr/include/c++/9/atomic:824:7: note: declared here
824 | atomic(const atomic&) = delete;
| ^~~~~~
/usr/include/c++/9/atomic:828:17: note: after user-defined conversion: ‘constexpr std::atomic::atomic(std::atomic::__integral_type)’
828 | constexpr atomic(__integral_type __i) noexcept : __base_type(__i) { }
| ^~~~~~
make[2]: *** [CMakeFiles/db_bench.dir/build.make:89: CMakeFiles/db_bench.dir/benchmarks/db_bench.cc.o] Error 1
make[1]: *** [CMakeFiles/Makefile2:245: CMakeFiles/db_bench.dir/all] Error 2
make: *** [Makefile:141: all] Error 2
```

I also noticed that travis has the same build failure: https://travis-ci.org/github/google/leveldb/jobs/753958518

Is there anything that I am missing?