Add rocksdb to the benchmarks

[jvoisin]

rocksdb has a neat benchmarking tool, and it written with hardware-efficiency in mind, so it should highlight some allocators shortcoming.

Moreover, it's buildable with a makefile, so integrating it shouldn't be hard.

[tvaleev]

The main question what tests are needed to add and run from rocksdb benchmarking tool? Currently I added: db_bench --benchmarks=fillrandom -- write N values in random key order in async mode

[jvoisin]

Looks good to me :)