Add fuzzy tests

[lukaszstolarczuk]

perhaps we should add some "fuzzy" tests, doing random operations on our kv database.

There are multiple choices for that; some of them include:

• the actual fuzzy testing framework (like google/oss-fuzz; inspired by Fuzzing axboe/fio#1160),
• rapidcheck generators, as we do in pmem/pmemstream repository,
• an approach similar to RocksDB tests: https://github.com/facebook/rocksdb/tree/main/fuzz